Well I don't know the official rules on this item, but I don't think it's what you are supposed to do.
The thing you do with a campaign is built a specific model for a specific campaign. So building 1 model for 2 campaigns doesn't really fit in that way of thinking if you understand what I mean....
So, I don't know the official rule but my own opinion is no.....
Join campaigns though, they're really fun!
And why not building 2 models lol :-)
Cheers,
Martin