The other way to do it is use a USB TTL and plug both into the EmonTX - again there has been discussion about this method.
The drawback (IIRC) is that you need to bind a specific USB device to a specific port so after a reboot the inputs are not switched.
[edit]
Yes, look at this thread. Using 3 No. EmonTX with EmonBase - #24 by AntiGrav