According to this link http://en.allexperts.com/q/Security-Fire-Protection-2247/Smoke-alarms.htm
they need to be on the same circuit. However, I don't know what qualifications this person possesses.
You could also try firing off an e-mail to this Firex technical support link http://www.icca.invensys.com/support/techsupport.htm

Good luck.

Shawn.