Not exactly what you're asking, but worth mentioning: I have stacked multiple Renogy DC-DC chargers in a dual alt Transit for 100 amps of house bank charge at idle.
Currently installing a second Renogy DC-DC into a dual-alternator Econoline ambulance for 110 amps of house bank charge. I have also previously installed two Victron Orion-Tr 15 amp 12v-24v DC-DC boxes into a single-alt 220 amp Sprinter for a tiny 30 amp charge to the 24v house bank. No reason the Victron units couldn't be used in the same scenarios.
In the case of a factory dual-alt Transit, there is no priority that I've found with either alternator and they are both controlled in the same manner; also the electrical load management control scheme is changed/removed already so you shouldn't have any issues. Keep in mind a good rule of thumb for charge system loads is 50% of claimed maximum alternator(s) output. 60% is give-or-take what they'll actually produce at idle and at operating temperature, which usually gives decent headroom for any factory charging/loads.