I added a 750 TI to my computer with a 1070, set <use_all_gpus>1</use_all_gpus>, and after a few tasks both GPUs did a task concurrently. However, afterwards, only dev 0 (1070) is doing any work, and there are GPU tasks ready to start.
How do I fix this?

Similar problem. I have RTX2060 and added today Nvidia P106-090. No luck wit latter:

06/04/2020 22:17:33 | | Running under account User
06/04/2020 22:17:34 | | CUDA: NVIDIA GPU 0: GeForce RTX 2060 (driver version 445.75, CUDA version 11.0, compute capability 7.5, 4096MB, 3552MB available, 6739 GFLOPS peak)
06/04/2020 22:17:34 | | CUDA: NVIDIA GPU 1 (not used): P106-090 (driver version 445.75, CUDA version 11.0, compute capability 6.1, 3012MB, 2965MB available, 1960 GFLOPS peak)
06/04/2020 22:17:34 | | OpenCL: NVIDIA GPU 0: GeForce RTX 2060 (driver version 445.75, device version OpenCL 1.2 CUDA, 6144MB, 3552MB available, 6739 GFLOPS peak)
06/04/2020 22:17:34 | | OpenCL: NVIDIA GPU 1 (ignored by config): P106-090 (driver version 445.75, device version OpenCL 1.2 CUDA, 3012MB, 2965MB available, 1960 GFLOPS peak)
06/04/2020 22:17:34 | | Host name: DESKTOP-L4Q76AT
06/04/2020 22:17:34 | | Processor: 12 GenuineIntel Intel(R) Core(TM) i7-5930K CPU @ 3.50GHz [Family 6 Model 63 Stepping 2]

Elevator Startups,

Your startup log shows that your second gpu is ignored by config. You probably need to add the <use_all_gpus>1</use_all_gpus> statement to your cc_config.xml file.

As I recall the default cc_config.xml is pretty useless. I copied this one from a long time ago and I believe it has every command there is. I just customize the ones I know for sure and leave the others alone:

<cc_config>
    <log_flags>
        <file_xfer>1</file_xfer>
        <sched_ops>1</sched_ops>
        <task>1</task>
        <app_msg_receive>0</app_msg_receive>
        <app_msg_send>0</app_msg_send>
        <async_file_debug>0</async_file_debug>
        <benchmark_debug>0</benchmark_debug>
        <checkpoint_debug>0</checkpoint_debug>
        <coproc_debug>0</coproc_debug>
        <cpu_sched>0</cpu_sched>
        <cpu_sched_debug>0</cpu_sched_debug>
        <cpu_sched_status>0</cpu_sched_status>
        <dcf_debug>0</dcf_debug>
        <disk_usage_debug>0</disk_usage_debug>
        <file_xfer_debug>0</file_xfer_debug>
        <gui_rpc_debug>0</gui_rpc_debug>
        <heartbeat_debug>0</heartbeat_debug>
        <http_debug>0</http_debug>
        <http_xfer_debug>0</http_xfer_debug>
        <idle_detection_debug>0</idle_detection_debug>
        <mem_usage_debug>0</mem_usage_debug>
        <network_status_debug>0</network_status_debug>
        <notice_debug>0</notice_debug>
        <poll_debug>0</poll_debug>
        <priority_debug>0</priority_debug>
        <proxy_debug>0</proxy_debug>
        <rr_simulation>0</rr_simulation>
        <rrsim_detail>0</rrsim_detail>
        <sched_op_debug>0</sched_op_debug>
        <scrsave_debug>0</scrsave_debug>
        <slot_debug>0</slot_debug>
        <state_debug>0</state_debug>
        <statefile_debug>0</statefile_debug>
        <suspend_debug>0</suspend_debug>
        <task_debug>0</task_debug>
        <time_debug>0</time_debug>
        <trickle_debug>0</trickle_debug>
        <unparsed_xml>0</unparsed_xml>
        <work_fetch_debug>0</work_fetch_debug>
    </log_flags>
    <options>
        <abort_jobs_on_exit>0</abort_jobs_on_exit>
        <allow_multiple_clients>0</allow_multiple_clients>
        <allow_remote_gui_rpc>1</allow_remote_gui_rpc>
        <client_download_url>https://boinc.berkeley.edu/download.php</client_download_url>
        <client_version_check_url>https://boinc.berkeley.edu/download.php?xml=1</client_version_check_url>
        <client_new_version_text>BOINC</client_new_version_text>
        <disallow_attach>0</disallow_attach>
        <dont_check_file_sizes>0</dont_check_file_sizes>
        <dont_contact_ref_site>0</dont_contact_ref_site>
        <dont_suspend_nci>1</dont_suspend_nci>
        <dont_use_vbox>1</dont_use_vbox>
        <exit_after_finish>0</exit_after_finish>
        <exit_before_start>0</exit_before_start>
        <exit_when_idle>0</exit_when_idle>
        <fetch_minimal_work>0</fetch_minimal_work>
        <fetch_on_update>0</fetch_on_update>
        <force_auth>basic</force_auth>
        <http_transfer_timeout>300</http_transfer_timeout>
        <http_transfer_timeout_bps>10</http_transfer_timeout_bps>
        <http_1_0>0</http_1_0>
        <lower_client_priority>0</lower_client_priority>        
        <max_event_log_lines>2000</max_event_log_lines>
        <max_file_xfers>9</max_file_xfers>
        <max_file_xfers_per_project>3</max_file_xfers_per_project>
        <max_stderr_file_size>0</max_stderr_file_size>
        <max_stdout_file_size>0</max_stdout_file_size>
        <max_tasks_reported>1000</max_tasks_reported>
        <ncpus>-1</ncpus>
        <network_test_url>https://FFRF.org</network_test_url>
        <no_alt_platform>0</no_alt_platform>
        <no_gpus>0</no_gpus>
        <no_info_fetch>0</no_info_fetch>
        <no_opencl>0</no_opencl>
        <no_priority_change>1</no_priority_change>
        <os_random_only>0</os_random_only>
        <process_priority>-1</process_priority>
        <process_priority_special>-1</process_priority_special>
<proxy_info>
    <socks_server_name></socks_server_name>
    <socks_server_port>80</socks_server_port>
    <http_server_name></http_server_name>
    <http_server_port>80</http_server_port>
    <socks5_user_name></socks5_user_name>
    <socks5_user_passwd></socks5_user_passwd>
    <http_user_name></http_user_name>
    <http_user_passwd></http_user_passwd>
    <no_proxy></no_proxy>
</proxy_info>
        <rec_half_life_days>7.000000</rec_half_life_days>
        <report_results_immediately>1</report_results_immediately>
        <run_apps_manually>0</run_apps_manually>
        <save_stats_days>10000</save_stats_days>
        <skip_cpu_benchmarks>0</skip_cpu_benchmarks>
        <simple_gui_only>0</simple_gui_only>
        <start_delay>0.000000</start_delay>
        <stderr_head>0</stderr_head>
        <suppress_net_info>0</suppress_net_info>
        <unsigned_apps_ok>0</unsigned_apps_ok>
        <use_all_gpus>1</use_all_gpus>
        <use_certs>0</use_certs>
        <use_certs_only>0</use_certs_only>
        <vbox_window>0</vbox_window>
    </options>
</cc_config>

https://boinc.berkeley.edu/wiki/Client_configuration