Two Systems. One Monitoring Fee.

I am almost ready to purchase a SS if I can get some answers and hopefully, find if the anticipated cameras will work with the current base unit.
That said, if I can't get the SS to work with my pole building (metal siding), I would likely return the barns sensors. BUT, what if I then just add a separate SS in the barn? My cellphone dials out with no issues. Would I have to buy two monitoring packages, or would they combine them? I realize they are two separate systems and would have separate identities, but they are at the same address with the same owner.