Depending on the hardware type you may be able to connect multiple IR blocks in parallel to the same IR source. I doubt the 1/4" Y cable is the way to do this, usually you need to take the IR input(not the emitter outputs) and split it.
You can also use dual emitters from a single emitter output if the block has enough power. A Y-cable can achieve this or you can buy dual-head-emitters.
You can also get a connecting block that has an emitter-like IR input, check out the russound 857, others may make one. This allows you to take the emitter output from one block and feed it into another block. Be careful mixing brands here as the signal has to be demodulated and remodulated again. Better to split the unmodulated signal that feeds the connecting blocks.
Also keep in mind that there are many blocks with more than 5 emitter ports. It is almost certainly cheaper to get one for the customer than to bill for your time to make an improvised solution if this is possible.
