Hello,
i have a mini ITX Pc for games.
I bought a GT630 to use it only for BOINC /GPUGRID.net.

About the option:
1. You can choose which GPU you want to run at 100% for BOINC.
2. If you have 2 GPUS you can also set one or more of those GPUs to stop run BOINC when you start a game.
Let's say, you browse something in Internet, both GPUs run at 100% on BOINC.
Now you start a game because you want to play a game, so the 1 GPU is stop running BOINC and the second is still running BOINC. This would help lot.

I'm not quite sure, because a.t.m. I've only got 1 GPU running and deactivated onboard graphics.

But as far as I remember, you can set which application can use which GPU by NVIDIA System settings:
1.: Right click on the Desktop (just the background, no icon or so). A list will appear
2.: Select "NVIDIA Sytem settings" from list (green NVIDIA symbol). A Popup Window will appear.
3.: Go to the Left Panel: Select "3D-Settings".
4.: Go to the Right Panel: Select "Programm Settings" Tab (not Global!). This will show most known games & apps.
5.: Probably, you will now have to add the BOINC.exe to the app list, then select it to change it's options.
6.: When BOINC.exe is selected, in the scrollable window below is an option "CUDA GPUs" (which is set to "all" by default, that's the key!). Just check the GPU you want to use for BOINC.
7.: Then you'll have to select the game and assign the other GPU to it.

I know, that this will only help for the situation "BOINC + game running simultaneously".
But I don't know if you can set a dynamic use (use 2 GPU for BOINC as long as no game is running) this way.

What you could test with NIVIDIA settings is:
Allow BOINC to use 2 GPUs, allow the game just to use 1 specific GPU.
Maybe this way the game will "borrow" 1 GPU from the 2 GPUs used by BOINC when it is started.

So, maybe at first try, assigning just 1 instead of all GPUs for the game is done more quickly (because most games are in the list, BOINC has to be added).