I did a design and installation earlier this year combining solar charging, alternator charging, and even shore power.
https://diysolarforum.com/threads/design-of-agm-lifepo4-solar-system-for-my-boat.78139/
Assuming that you have lead-acid batteries for the engines, you will want to isolate the engine batteries from the house battery bank (your new LiFePO4 batteries). A dc-dc charger should do the trick.
While the engines are off (and away from shore power) you will rely on the capacity within the house bank and whatever the solar charging can deliver. The EPEver DuoRacer series of solar controllers have 2 outputs, which can provide solar charging to the house bank and trickle charging to the engine battery bank. So, you may want to consider the DuoRacer.
I also used the Victron Orion series of DC-DC converters, which has the ability to sense when the engines (alternators) are running, and can also be activated by an ignition switch feed.
What I learned throughout the design/engineering process, is that the DC-DC charger likes to be connected to an engine battery for 'stability' (and power source), but you need to have a way to de-activate the DC-DC charger when the engines are not running, so as not to run the engine batteries down. The Victron Orion provided such features. A friend of mine did a similar installation, but used a manual switch at the helm to activate/deactivate the DC-DC charger. This approach certainly would work, but it has the risk of forgetting to turn the switch off.
I also found that with several 'intelligent' chargers involved in the system, they don't always like to be activated at the same time (i.e. running in parallel). It is not overly harmful, but I found that the system would see occasional over voltages as the various chargers were each trying to do their own thing.
