At my barn we have tried them all. The winner?
This cheap set from Amazon meant for referees and motorcyclists.
https://www.amazon.com/EJEAS-Professional-Motorcycle-Snowmobile-Interphone/dp/B071S32RW7/ref=sr_1_4?ie=UTF8&qid=1534551821&sr=8-4&keywords=referee+intercom
For $120, you can get 2 units.
If one breaks, simply replace it.
Many of the equestrian sets are more expensive and only come in sets of 2.
Here are the ones we tried previously:
Eartec Simultalk - used to be sold at Dover. Lasted ~6mos then crapped out.
OneK, with the rider helmet insert - this one lasted about a year, but it was gross to swap it between riders in back-to-back lessons, and cost prohibitive for each rider to have their own.
CeeCoach – Expensive and fragile - lasted roughly 6mos, the earbuds don’t fit everyone super well