Sounds like that will work for you!
I'm running a dual battery/dual alternator setup in which both systems are completely isolated from each other during normal operation (except when cranking the engine), with a small third backup battery to handle power interrupts and/or run essential equipment should both buses somehow fail. I had considered TCW's products as well as a few others, but none of them did exactly what I wanted, so I decided to DIY. It's been a long road and we're nowhere near done yet haha.
I designed a circuit that would take a power input from either Bus 1 or 2 (selectable via a toggle switch) and feed it through a high-current Schottky diode array (VS-115CNQ015APBF) out to the E-Bus. The other side of the diode array is powered by the standby battery, which will automatically source the E-Bus if the primary power source fails.
The board actually has three blocks similar to described above:
1. A "backup" bus that powers my entire "Avionics 2" bus. This can be switched off via an external switch to prevent excessive draw, but when turned on will be able to power almost all of my avionics including all displays etc during engine cranking.
2. An "essential" bus that powers a few essential components and a standby flap/trim circuit
3. Using a smaller-rated FERD40U50C diode array, this backs up one of my LSE ignitions, which has its power sourced separately (they connect straight to the battery).
The Hall Effect current sensor I mentioned earlier (ACS758LCB-100B-PFF-T) is connected inline with the battery input, and measures bidirectionally so I can see if the battery is charging or discharging. I can even set up an alert on the G3X to tell me if the standby battery is draining, so I will be aware if something isn't working correctly.
Since the current sensor needs 5V DC to operate, I also needed to include a small power supply on the board that uses a simple 78L05 linear voltage regulator, with other supporting circuitry.
In use, there are three toggle switches associated with this system, (1) a stby battery master switch, (2) an e-bus source to select Bus 1/2, (3) Backup/Essential mode only (this disconnects the Backup bus from the stby battery).
I've got all this laid out on a PCB and sent the design to OSHPark to fabricate a board for me. I expect to receive it sometime this week or next, and can keep you posted on progress!