How much do you gain from minting a proof-of-stake coin on multiple wallets? Is there a theoretical gain from mining the same wallet/coins for pos on multiple computers, and if so, how much is it?
There is no theoretical gain. It would basically be multiple computers doing the same calculations and arriving at the same result. 

Look at https://github.com/ppcoin/ppcoin/blob/master/src/kernel.cpp#L290-L312. These are the things that are included in a PoS hash:

//   nStakeModifier: 
//       (v0.3) scrambles computation to make it very difficult to precompute
//              future proof-of-stake at the time of the coin's confirmation
//       (v0.2) nBits (deprecated): encodes all past block timestamps
//   txPrev.block.nTime: prevent nodes from guessing a good timestamp to
//                       generate transaction for future advantage
//   txPrev.offset: offset of txPrev inside block, to reduce the chance of 
//                  nodes generating coinstake at the same time
//   txPrev.nTime: reduce the chance of nodes generating coinstake at the same
//                 time
//   txPrev.vout.n: output number of txPrev, to reduce the chance of nodes
//                  generating coinstake at the same time


These will be the same on two different wallets, so you'll just be doing the same calculation and getting the same result on two different computers.


  One possible theoretical gain is that blocks could be propagated from different nodes at many locations, and the blocks would spread much more quickly.