How to assign just 1 cpu to 2 GPU's if possible.

In your thread title, you ask about 'assigning' 1 CPU to 2 GPU tasks.  You cannot assign - rather you create a budget for BOINC to use when it decides what mix of tasks to run so as not to over-allocate the available resources.

The thing that's really important to remember is that any GPU task will probably need some level of support and you shouldn't try to force it to take more or less than it actually uses.  Project supplied values are only estimates and may be wildly inappropriate values to use, depending on individual circumstances.  They absolutely can't account for complicated project mixes.

Also how frequently the resource needs occur (and the latency in servicing them) can have a dramatic effect on GPU task performance.  In simple terms, if a GPU task has to 'fight for access', its performance could suffer really, really dramatically.

If you haven't already done so, you should find out for yourself, exactly what you will lose in GPU task performance by running CPU tasks on all threads.  It's very easy to do.  Just suspend all CPU tasks on board and allow just a single GPU task to run (choosing just one - suspend all others) so you find the absolute best possible crunch time for that type of task with nothing else competing.  Rinse and repeat for each different type of GPU task.

Then do exactly the same again but with all 16 CPU threads running CPU tasks.  When you finish that, you will have a very good idea of what you will lose in GPU performance in the worst case scenario of all CPU threads being fully used.  Since you state that you always run all cores, you've pretty much done this test already.

If you were really keen to optimise performance, you could rinse and repeat with 15, 14, 13, etc. threads running to see where the optimum mix between CPU output and GPU output really was.  When you have found that number, you can set it by changing the % of cores BOINC is allowed to use.  For example, if you found the optimum conditions were with 13 CPU tasks running, you would just set BOINC's % of cores allowed to something like 82% (anything above 81.25% but not as high as 87.5% which would only reserve 2 cores).  I suspect that optimum conditions might be with more CPU cores reserved than the 3 in the example.

Alternatively, you could have an independent (but much more complicated) method of control if you decided to use the app_config.xml method that Harri has pointed you towards.  If you have several different GPU projects, that could be quite difficult to get right, since each one would need a carefully thought out app_config.xml file of its own.

Because you have such a complicated mix of projects to run, and your own particular hardware mix to run them on, nobody will be able to give you any sort of accurate answer.  You have to work out the best solution for your particular circumstances.

With 16 CPU tasks running, the GPU task is having to fight for what it needs.  There is no magic cavity somewhere with 0.144 CPUs sitting and waiting, ready to go.  When the GPU needs CPU support, it will be the OS (not BOINC or the magic cavity) that will supply it.  There will be a queue and that queue will likely be lengthy because all CPUs are heavily in use all the time.  This is precisely why you need to do the experiments to see what you will lose by joining a long queue.

You need to realise that if all your 0.144 CPUs (or other fractional values in play at the time) don't add up to a full core, they may as well not be there at all.  BOINC will still run the full number of CPU tasks allowed, even if all these fractional budgets added up to 0.999 CPUs.

Fractional budgets are pretty useless.  You really need to see for yourself what you lose if you don't have BOINC reserve a full thread even if the budget claims that 0.144 (or 0.9) is all that is needed.

Please also realise that your machine has 8 cores (16 threads).  That means you already have 2 CPU tasks fighting each other for access to a single core.  That may be very beneficial if each competing task needs different parts of the core.  That's probably much less likely if tasks are from the same project and both running the same sort of high intensity number crunching.  So a GPU task may be quite hampered in getting access in a timely manner when it needs to.

Nvidia owners have repeatedly noted on these boards and for quite a while that an nvidia GPU task actually needs significantly more than a full thread and that if it's not immediately available, performance does suffer.

You have two GTX 1080 GPUs (according to the website).  I looked at some crunch times, many were several hours (many examples in the 2-4 hour range) which is absolutely woeful compared to what that hardware could do if it had proper CPU support.

Do the first basic test listed above and give yourself quite a shock as to what the proper crunch times could be.

I usually set my CPU % to 87.5 with 100 % usage for my 8c/16t cpus.   I have a 2700x, 3950x, and another CPU.

And lower the CPU threads total for all projects to at least 1 thread lower.

In your case, I would guess you want to run no more than 13 cpu threads.

Tom M