I used to do the same thing with PCBFast, and often get away with it, but the last 2 jobs, they cracked down - so if it looks REMOTELY like it is not just 1 design, they charge for multiples !! Many emails later, they refused to back down. On one occasion, I added tracks linking the pcb's - nope, didn't accept it.
Sure peed me off as often it was just little adapters with 2-3 small connectors and a 1/2 doz damn wires - couple squ cm. Maybe i'll try that with JLC next :-)