Yup... The ET73's will interfere with each other. Habs has you pointed in the right direction... Use an ET73 and an ET7 as they operate on different frequencies.
The ET7 operates on 433.92 MhZ.
The ET73 operates on ??

For some reason the company web site, instruction sheet, and packaging doesn't say what frequency the ET73 transmits at... That's kinda odd.
Unless of course... I'm blind this morning.

Mike