Many states are requiring both sensors now in their fie safety laws. All major detector companies sell nice little dual units. COVE sells a dual unit now for their very similar system. When I signed up for Simplisafe the rep told me 'you were working on having one' which I gather was not really true, but it did make the sale.
It seems like such a simple thign to do, which would make you lots of money in unit sales. As well as keeping up with what COVE is doing. SO why don't you make one?