Here's my 2 cents:
Technically it would work since you are basically using the 15G as a sump for the 10G. Two key issues is to check flow rate at head pressure associated with your system which is usually measured in feet of head. I think this would be the distance between the surfaces of each tank, but slipknottin would know for sure. Powerheads are usually designed for recirculation and may not handle much head pressure. And as with any sump, if you have a power failure, be sure the water level in the 15G tank is low enough to handle the volume of water in circulation.
On a practical side, it seems like a lot of hassle for two small tanks when a heater and a small AC powerfilter are relatively inexpensive. The extra expense of a second heater and filter is (IMHO) offset by advantages in redundancy and disease control. Both tanks aren't at risk if a component fails. Also, fish in one tank are not exposed to diseased fish in the other tank should an outbreak occur.
Good luck.