My MacPro 5,1 has two NVIDIA GTX 1070 GPUs. One is connected to the internal power supply. One is connected to an external power supply. I can connect either GPU to a monitor (hot swappable). See links for photos:
https://www.dropbox.com/s/vmpyyuh60l9k9ak/Config%201%20Slot%201.jpeg?dl=0
https://www.dropbox.com/s/2en0cyf2hi4an3y/Config%201%20Slot%202.jpeg?dl=0

However, BOINC only recognizes one of the two GPUs. I don't have any problem getting BOINC to recognize two NVIDIA GTX 750Ti GPUs. Any idea on how I can get BOINC to recognize the second GTX 1070? Thanks in advance for any advice. Log file excerpt is below:

Thu Mar 5 20:06:25 2020 | | Starting BOINC client version 7.14.2 for x86_64-apple-darwin
Thu Mar 5 20:06:25 2020 | | log flags: file_xfer, sched_ops, task
Thu Mar 5 20:06:25 2020 | | Libraries: libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 c-ares/1.13.0
Thu Mar 5 20:06:25 2020 | | Data directory: /Library/Application Support/BOINC Data
Thu Mar 5 20:06:25 2020 | | OpenCL: NVIDIA GPU 0: GeForce GTX 1070 (driver version 10.33.0 387.10.10.10.40.134, device version OpenCL 1.2, 8192MB, 8192MB available, 1712 GFLOPS peak)
Thu Mar 5 20:06:25 2020 | | OpenCL CPU: Intel(R) Xeon(R) CPU E5520 @ 2.27GHz (OpenCL driver vendor: Apple, driver version 1.1, device version OpenCL 1.2)
Thu Mar 5 20:06:25 2020 | | Host name: DJR-Home-Office-MacPro-51-1.local
Thu Mar 5 20:06:25 2020 | | Processor: 16 GenuineIntel Intel(R) Xeon(R) CPU E5520 @ 2.27GHz [x86 Family 6 Model 26 Stepping 5]
Thu Mar 5 20:06:25 2020 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clfsh ds acpi mmx fxsr sse sse2 ss htt tm pbe pni dtes64 mon dscpl vmx est tm2 ssse3 cx16 tpr pdcm sse4_1 sse4_2 popcnt
Thu Mar 5 20:06:25 2020 | | OS: Mac OS X 10.13.6 (Darwin 17.7.0)
Thu Mar 5 20:06:25 2020 | | Memory: 48.00 GB physical, 667.06 GB virtual
Thu Mar 5 20:06:25 2020 | | Disk: 930.71 GB total, 666.82 GB free
Thu Mar 5 20:06:25 2020 | | Local time is UTC -6 hours
Thu Mar 5 20:06:25 2020 | Rosetta@home | Config: excluded GPU. Type: NVIDIA. App: all. Device: all
Thu Mar 5 20:06:25 2020 | collatz | Config: excluded GPU. Type: ATI. App: all. Device: all
Thu Mar 5 20:06:25 2020 | | cc_config.xml: bad URL in GPU exclusion: http://einstein.phys.uwm.edu/
Thu Mar 5 20:06:25 2020 | | cc_config.xml: bad URL in GPU exclusion: http://setiathome.berkeley.edu/
Thu Mar 5 20:06:25 2020 | collatz | cc_config.xml: bad type 'ATI' in GPU exclusion; valid types: NVIDIA
Thu Mar 5 20:06:25 2020 | collatz | URL https://boinc.thesonntags.com/collatz/; Computer ID 850946; resource share 40
Thu Mar 5 20:06:25 2020 | Rosetta@home | URL http://boinc.bakerlab.org/rosetta/; Computer ID 3700872; resource share 20
Thu Mar 5 20:06:25 2020 | | General prefs: from http://boinc.thesonntags.com/collatz/ (last modified 09-Oct-2014 08:54:01)
Thu Mar 5 20:06:25 2020 | | Host location: none
Thu Mar 5 20:06:25 2020 | | General prefs: using your defaults
Thu Mar 5 20:06:25 2020 | | Reading preferences override file
Thu Mar 5 20:06:25 2020 | | Preferences:
Thu Mar 5 20:06:25 2020 | | max memory usage when active: 24576.00 MB
Thu Mar 5 20:06:25 2020 | | max memory usage when idle: 44236.80 MB
Thu Mar 5 20:06:27 2020 | | max disk usage: 674.41 GB
Thu Mar 5 20:06:27 2020 | | max CPUs used: 13
Thu Mar 5 20:06:27 2020 | | suspend work if non-BOINC CPU load exceeds 25%
Thu Mar 5 20:06:27 2020 | | (to change preferences, visit a project web site or select Preferences in the Manager)
Thu Mar 5 20:06:27 2020 | | Setting up project and slot directories
Thu Mar 5 20:09:42 2020 | | Checking active tasks
Thu Mar 5 20:09:42 2020 | | Setting up GUI RPC socket
Thu Mar 5 20:09:42 2020 | | Checking presence of 2610 project files

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I have diagnosed the problem. MacOS 10.13.6 High Sierra does not load OpenCL for the second GTX 1070, even though the System Profiler acknowledges the presence of the second GPU and the second GPU can successfully drive a monitor. However, MacOS 10.12.6 Sierra DOES load OpenCL for the second GTX 1070 and BOINC is capable of utilizing both GPUs.

It is possible that this "bug" is specific to 10.13.6, as my BOINC recognizes two AMD RX 580 eGPUs connected to an iMac 18,1 running MacOS 10.14.6 Mojave. But, the conspiracy theorist in me believes the "bug" is specific to the combination of MacOS beyond Sierra and NVIDIA GPUs, as it is well evidenced that Apple and NVIDIA are no longer cooperating as they once did. More importantly, eGPUs that utilize the NVIDIA GPUs with Maxwell architecture and beyond are officially incompatible with Macs.

It's a pity, as there are some really great NVIDIA GPUs that I would love to run in my MacPro 5,1 boxes or as eGPUs connected to my more contemporary iMacs, Mac Minis, and MacBook Pros.

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Since apple is based on Unix, which is very close to Linux,
In Linux there's a command (sudo nvidia-xconfig --enable-all-gpus) that will enable more than 1 Nvidia GPU.
By default even in Linux, only 1 GPU is recognized. And in 18.04 or older, this command works well, but in newer operating systems, this command gives desktop errors.