Would it cause any problems, either for the network or the miners if I solo mine from 2 different computers, on different ISPs but using the same wallet?
You don't really need to have simplewallet open when you mine. Your blocks are stored in the blockchain and will update next time you open the wallet. On your second computer, just run bipcoind like this:
bipcoind --start-mining YOURADDRESSHERE --mining-threads 4
Make sure you set your address and your preferred number of threads. I'm pretty sure you could run it like this on 100 different computers without causing any problems.
If mining on two copmuters to one wallet works, let us know. And also if it doesn't. FYI, I'm pretty sure you'll need these files from computer A also copied to your BipCoin folder on Computer B:
(wallet name).wallet
(wallet name).address
I have that and I can check one wallet's balance from another computer.