No need for extra diodes. IR emitters used by Xantech, Crestron and almost everyone else are infra red liight emitting diodes (IR-LEDs). That's why you always have to watch their polarity during installation.
Wire them in series (positive side of first emitter to signal out/tip, negative side of first emitter to positive side of second emitter, negative side of second emitter to ground/sleeve.
Have done it often and works always.