-------------------------------------------------------------- Quote from spindr0: Why would one do this in separate accounts in the first place? -------------------------------------------------------------- I think that's a winning answer!
It has nothing to do with the stocks being fungible. You have to look at the process behind it. Both IB and MF are clearers so essentially you are talking about transfering a stock between two clearing firms, which is possible, but I doubt anyone would go through the trouble of doing it for 100 shares and it may take a while, so my answer would still be: "no, it's not possible".
I don't know current policy since I haven't opened a new account in a long time but 10-15+ years ago, many brokers began charging a fee of $50 or more to ACAD a stock position (or account) out.