Great feedback on the cell modem. Enphase will wave the requirement if you make a convincing case, like no cell service in your area!
Testing was done on AC Out. Will try AC1, should work by connecting AC2 to the micro-grid and we'll get EPO disconnect for free with the AC1 relay. The XW Pro will still produce power on the AC (load) lines, but they are not the be used anywhere. Actually, the AC (load) lines will give you UPS power no matter what the EPO dictates, but local fire code may not allow this mode of operation. The XW Pro behaves like a grid-tied inverter that should disconnect AC1 when the micro-grid voltage goes down, continuing to produce power on AC (load). Using AC2 (Gen) is a work-around by SE to use the AC2 grid sensing transformers because they have no isolated AC Grid voltage sensing transformers available on the AC relay board, so SE sacrifices the Gen AC2 port for grid sensing.
If you stack only 2 XW Pro's you won't need an external contactor. With the built-in surge capability of the 2 stacked XW Pro's, they can route/make up to 120A by parallel stacking. The built-in XW Deltron relays will need to carry that much current only under extreme backup load and very low Enphase SOC. Your main goal is to supplement/augment battery energy/storage and provide surge capability, both of which Enphase can't do beyond your maxed out system. Unless this extreme surge happens several times per day with EPO disconnect happening at the same time, you won't have any problems with XW Pro relay arcing. You are using the XW Pro as an AC battery, relays connected all the time, except during an EPO emergency shutdown. The more I analyze your setup, the more I think you could stack up to 4 units without an external contactor at all.
A Note of Caution:
You basically have two independent backup systems, that synchronize themselves if switched on in the right sequence.
Case 1: Enphase is switched on first, becomes the micro-grid reference source, XW is connected second, will micro-grid-tie and sync to the Enphase primary AC voltage, inject power into the micro-grid depending on XW battery SOC. Enphase is the grid generator, XW is the grid follower. This should be your "normal" mode of operation.
Case 2: XW Pro is switched on first, becomes the microgrid reference source, Enphase is connected second, will micro-grid-tie and sync to the XW Pro primary AC voltage. XW is the grid generator and Enphase is the grid follower. XW Pro may perform FW to curtail power production. Not sure if it will work with no load and not sure how Enphase backup will respond. Not sure if it will work at all.
Case 3 - Caution: Disconnect the XW Pro breaker and switch on both systems. They both produce AC, but unsynchronized. Connect a volt meter or a 240 V Neon indicator between Enphase L1 and XW Pro AC(out) L1, basically across the open breaker. You'll see voltages slowly vary from 0 to 240 V and the Neon light vary dark to bright as the phase between the 2 systems shifts in and out of sync. You might consider installing a red Neon light permanently just to be safe, if it's red the 2 systems are out of sync.
When the voltage difference is 0 it is safe to connect both systems, when it is at 240V you could trip the breaker or worse damage the inverters.
Always switch off the XW Pro before disconnecting the breaker. Close the XW Pro breaker and then startup the XW Pro.
You are setting up a very interesting system to augment and extend an Enphase backup system with non-Enphase backup configured as AC batteries. Should also work with other hybrid battery backup inverters like Outback, Tesla Powerwall, FranklinWH, EP Cube, Sol-Ark, LUX Power, SunGoldPower, GroWatt and others.
