I used a n800 and n82 configured with the same SIP account. The n82 disconnected in standby mode. Since I shutdown the n800, the connection in standby is stable.
I think we can consider it's a bug of the s60 (the n800 never had problems but it was also configured first). In fact, more than one person can use the same account.

I have a N82 and an E51 configured and connected to the same sip account (in my case Sipgate) works fine.

The only service you may possibly have problems with is Truphone who link your account to a GSM number, however there are way round that too :)

Hey RatKat, can you share us the procedure to handle the TruPhone SIP profile to be usable on multiple symbian devices ? I know its currently tied on to the GSM/Sim. Please let us know.

Cheers,
Ravi.