What isolates your engine alternators from the other engine's start battery or the house battery? Whatever provides that isolation can do the same for the solar output. In other words you should think about isolation globally and use that same device for your solar output. For example if one of your start batteries charges the house bank directly and a combiner/isolator/??? charges the starting battery when the voltage is right, then use that scheme and connect your solar input to the alternator's output so it will be treated the same.
I won't go any further with this line because it is too complicated, particularly with twin engines to make any sense over the internet, but I will talk a bit about isolation in general:
There are several ways:
Diode isolator- Bad because most cause a voltage drop.
Simple combiner- Yandina and others make simple voltage sensitive relays. Good, but lets the two combined batteries share the same voltage from the charging source. Some (not me) think that this is bad.
ACR, automatic charging relay- In general these are just an electronic version of the relay device above.
Balmar's Digital Duo Charge- This one and others like it (don't know of any, but I will bet that there are some) provide isolation like the ACR but also have internal circuitry that limits the voltage profile supplied to the starting battery (usually the secondary one in a combiner setup). FLA, AGM and Gel profiles are available.
So look at your solar output as another charging source that must be combined and isolated from the others. If you already have a working isolation system, then just hook your solar output to your house bank. That is the one that needs it the most and it would be rare that your starting batteries needed it. This is the simplest solution of all.
David