Even though you have dual alternators, is there still a maximum amperage you can get out of a CCP2 site under he seat to use to power a Battery to Battery charging unit for the house battery?
Thanks!,,,,Short answer to your question: 175A max; 150A realistic; and it /might/ draw that even on idle - and /maybe/ even when the engine auto-shuts-off.
Blending the above posts and adding our observations:
Assuming that calling it, "CCP2," means you have a 2020+ unit. Previous to 2020, there was either one or three identical 60A fused terminals; 2020+ have either only one 60A or one 60A and one 175A fused terminals.
That 175A fused terminal is what many of us have used with a 150A circuit breaker - logic being better to pop an easily accessible breaker than dig under the seat for that 175A fuse. FWIW, CCP2 also has a 190A relay on it; it turns on/off with engine on/off - but does NOT turn off when the engine turns off at a stoplight, for example.
When the engine turns off and there's a big load on CCP2, the voltage will drop a LOT - to under 12VDC. This causes problems with many / most of the DC-DC units as they are often triggered (by default config) on voltage; many of them shut off when they see the drop in voltage. Some folks have found ways around this with some installs; others seem to still have troubles even with the same equipment. The all-in-one / solar-generator units seem to have more trouble with this from reports on here - and often seem unfixable for some folks.
Our experience with our 2020 has been quite positive with pulling 150A regularly from CCP2 - even at idle. I can't speak to why our setup works; but it has worked for a couple other people who have done the same setup. We're running a DC-AC-DC setup to charge our main batteries: there is an inverter attached to CCP2 and that inverter drives the Multiplus inverter-charger, which charges the batteries. We're able to limit the charge rate from zero to 50A - so we keep it at 13A @ 120VAC, which is ~150A @ 12VDC. The main reason I'm detailing this is that we consistently get 150A at idle - though the voltage drops down to mid-11s at times; thanks to dual starter batteries, it's never been a problem.
The reason I'm pointing that out - based on observed / measured data for over a year - is that it's inconsistent with what @ArgonautVans comments above: we actually get 150A at idle - and even when the engine turns off. It may also be worth noting that the "smart alternator" aspect of the Transit means that the voltage may not be ~14+ even at highway speeds or on idle depending on what the system thinks it "needs" - meaning it will actually drop voltage even with the engine ON at idle.
SO... it's a bit of a mess compared to old-school simple stuff.
So your next question should be about what methods of DC-DC (or DC-AC-DC) will work best for you or not. 😏