Soundman, welcome to the forum!
We have a job with the following that needs to be tripped by 2 epo buttons
GE Main Breaker - 2 black wires
Siemens 50A 2 pole breaker- Breaker has 3 pole with 1 white wire and one terminal.
2 HVAC units with 24vdc trip contacts
2 epo buttons with N/O / N/C contacts
what is the best way to get this all wired up?
If either button must trip, they simply get paralleled. Supply 120v from a 1p "shunt-trip" breaker (unless you already have an appropriate control circuit) to 'line' side of the pair of buttons.
The 'load' side of the buttons connects to either black wire of the GE breaker and the terminal of the Siemens 2p. (Are both breakers in the same panel? If so the Siemens would be redundant.)
The other GE black wire and the Siemens white wire connect to the neutral bus of the same panel the new 1p breaker is in. Ideally, this 1p breaker is in a panel protected by one of the shunt-trips.
(A shunt-trip breaker has a solenoid that trips the mechanism by remote control. Some shunt-trip breaker's can't withstand having their coils continuously energized.)
If the low-voltage DC tripping units are powered, and just need a dry-contact closure to trip, a 1- or 2-pole breaker with a 120v coil wired in parallel with the shunt-trip breaker coils should work.