Thanks Richard.

If anyone tests this, post your findings.

You're right. Theoretically. :)

There is a prerequisite for this scenario: Your PC should not keep a spare pair of WU's in it's queue for too long, because if these two spare WUs are sitting in the queue for 12 hours, and then they are processed for another 12 hours they will just miss the 24 hours deadline. But if your queue is short, and a WU fails, your GPU will run only one WU (or in worst case: nothing) until a new one (or two) is downloaded.

You forget about the reason behind low GPU utilization:
GPUGrid uses both the GPU and the CPU for processing a WU. The more the CPU usage of a WU is, the less its GPU usage is, because of the heavy data transmission on the PCIe bus. A GIANNI_KKFREE WU (82% GPU usage) uses 5.5 times more CPU than a TONI_AGGsoup WU (98% GPU usage). I learned from my experience, that I have to overclock a Core2Duo by 33% to get the performance (say 5-10% more GPU usage) of a Core i3 (its integrated PCIe controller is 33% faster than the X48 chipset's). I think the existence of a bus between the GPU and the CPU is the bottleneck, and this bus (ie. the PCIe) can be overloaded even by processing a single GIANNI_KKFREE-like WU. If this is right, there will be no significant rise in GPU usage by running two of them simultaneously (although there can be significant rise in GPU usage by running low and high GPU utilizing tasks at the same time, but maybe the lower GPU utilizing would be more slow than expected).
Don't get me wrong, I'm curious about this, but at the same time I'm very skeptical. Let's find out...

Try this message from the Einstein forums. It has an app_info.xml used for running 4 Einstein tasks on a GTX480. It should give you the general idea. Maybe you can modify it to work with GPUgrid.

If you installed BOINC from repositories then it is NOT setup to run on your account. It will run on a special boinc user's account which is usually named boinc or something similar. In that case the boinc user will not see the SWAN_SYNC environment variable if you put it in your .bashrc. If you installed BOINC from the Berkeley installer (the .sh script) then putting it in your .bashrc is adequate.

It only says so if you are running SWAN_SYNC, it doesn't mention anything if not (at least when looking at my results).

Be careful when putting SWAN_SYNC in .bashrc, it will only work if boinc is running as that user.
To set it system-wide, look in /etc/environment or /etc/env.d/

It might be on by default in Linux, I don't know. The stderr on the website for each task shows the value for SWAN_SYNC. If it says there the value is 0 and you have no "export SWAN_SYNC=0" statement anywhere then assume it's 0 by default. I put "export SWAN_SYNC=0" in ~/.bashrc. Note the period, it's a hidden file. I'm the only user on the system so that's adequate for me. If you have multiple users you'll want to put the statement in a script that runs for every user.

Yah, nvclock doesn't work here either. The nvidia-settings utility doesn't give GPU load either. I've been able to read GPU usage only with the nvidia-smi utility and only with the 270.xx drivers. It doesn't report usage here with the 260.xx driver.

There's only one way to be sure and that's to try it and see. I'm sure skgiven and others have been down that road before so keep in mind what they say when you're testing. And as Retvari said, make sure your tasks return within 24 hrs. if you want the credit bonus. I think a GTX 570 will have no problem returning 2 concurrently running tasks in 24 hrs. but ya never know.

Also, I had troubles with the temperature on my GTX570 when I started running it on Linux. The fan would not speed up as the temp rose so I had to manually force it to run at high speed permanently. That required some research and playing around. I expect you and others will run into the same problem so I wrote up what I did and posted it here on the BOINC dev forum. I recommend getting a handle on that BEFORE you run your first CUDA task on Linux.