In trying to find the answer to the above question, I noticed 2 things. First, Google Suggest came up with that exact phrase as I typed my search terms, and second, there was no good answer when you searched it. I'm going to go ahead and assume others are looking for the same answer and also couldn't find it. So then I had to go with the empirical method and just try it myself.
And no. It won't let you.
Hopefully now people can find the answer ahead of time.
P.S. I then tried forwarding to the same number the other Google Voice number forwards to, and that doesn't work either. It'll let you go ahead and add the number, but then it kicks that number off your other account. Adding insult to injury, to get it back on your other account you have to re-claim it and set up voicemail forwarding all over again.