The only thing bad with emitters in series is when goes bad..both dont work..Although I have only had that issue one time in 15 years..or if you have a bad out of the box emitter, troubleshooting can be not so fun. Again have only had a few bad ones DOA

We usually use a Xantech CB60 with the RP-1, just use a Mono mini plugged into the CB60 with a long wire connected to the RP-1, no power needed and gives you 6 IR ports, all ports have a 100 ohm resistor in series on each port

I usually Hook it up like an Emitter expander Shown on page 2 of the istallation manual
[Link: xantech.com]