Whatever the tarp weighs will technically be carried by two people. A spinn 12x10 from oes weighs 16 oz. which comes to 8 oz a person. 8 oz doesn't sound like a heavy tarp to me.
I agree with Akers - and having done the two-person-under-one-tarp thing pretty regularly, I can undoubtedly say I'd hate to have less than a 12x10 for two people when the rain is falling hard and the wind is a-blowin'.
The only downside I see to the "8 oz per person" calculation is that, in the occasional circumstance where you can't find a set of trees appropriately spaced for two person hangs, it can be nice to have a second tarp as a backup.
About finding good trees, what if you were to use two trees and do bunk beds? Maybe putting the tarp on the 10' ridgeline?
It may not be rocket science, but it's still science.
TZ, I can't really see how the hiking poles are attached in the pics. Maybe a more in depth description or the cup and loop. I do like that idea though.
There are photos of 2 different attachments in this thread
Hope that helps