Ivan is pretty much right. The issue with solenoid switches is generally the coils are not continuously rated and let the smoke out when you least want it to.
You really need a very heavy duty relay - as big as you can find. Lighting relays up to 90A are available and this is plenty for your application.
Both car batteries and alternators are designed to work together and there is limited merit in having a managed charger / conditioning unit in there as well. I think most are sold because they are a bit bling - blue LED's and the like. The most I would do is fit a volt meter on the dash for each battery.
The normal split charge arrangement connects both batteries together whilst charging/discharging - which is fine for them.
If you just connect a pair of batteries in parallel, it is likely that one may have a little more charge than the other. It will pump charge into the other battery - but over-shoots a bit and the other battery needs to pump charge back - and it over-shoots too. This way, a small amount of charge is continuously cycled back and forth, flattening the batteries over time.
Give me a shout if you want a diagram, help or even a suitable relay!
Si