Going "on-line" from FTMM requires you to log on to your ACOM account. You can only be logged-on to your ACOM account from one FTMM program at a time.
I'm not sure what havoc it will wreak, but you could possibly log-off of your account on the computer with FTMM2, and then try and log-on to your ACOM account from the other computer with the older FTMM version.
If you have auto-sync turned on in FTMM2, I would def turn it off before trying this.
An ACOM member tree can only be linked to one FTMM tree/file.