Hi, I'm kind of surprised that they dont work in parallel, as your typical 3pin IR modules usually have a weak pulled-up output and I've seen modules being used in parallel though not for the reason you intend to. The circuit I'd seen parallel'ed multiple modules, each meant for a different carrier so that the receiver could get a signal from a range of carriers.
Anyway, in your case, I think Nandu's suggestion can be implemented by simply using 2 signal diodes (1N4148 or any); connect the cathode of each to each of the IR module outputs; connect the anodes together, and pull them high thru' a 10K resistor; this becomes your OR'ed composite output.
Havent actually tried this, but should work.
Regards,
Anand Dhuru