I wouldn't recommend sharing the lines for fire alarms. Like Merritt said one will over ride the other. I seriously doubt any Fire Marshall would approve of that if they were aware of it.
Is it possible to connect the two panels together and use one to report signals? That is fine in many jurisdictions (in Texas anyway).

Jeff, I've seen burglar alarms share fire alarm lines but it's not a good situation for the same reason as the fire alarm situation but it is the lesser of the two evils.