Basically soldering wires together is a good idea. But just as with welding you either got to do it right or you should resort to another type of bonding (something with screws for example).

Connection between alternator and both batteries. Unfused.

I’m not quite sure how much current the gel acid battery can pull but certainly more than 50 amps. The wires are 5mm in diameter. A good joint is recognized by solder flowing in between the little wires and a flat & shiny surface. That’s because solder really flows like oil if it has the right temperature. Here the solder is only dabbed on top. To get a nice connection you have to heat up the copper to the melting point of the solder (close to 400 degrees Celsius). Since copper is really good in conducting heat you would need something like a little blowtorch to get such a cable to the right temperature. This would also ruin the insulation. In short: Do not try to solder thick wires. Use terminals. And don’t forget the cable end sleeves on screw terminals. They save lifes! If you just screw the wire in you will damage the little wires and by time the connection is so thin that it heats up and / or get’s disconnected. Since the whole body of a car is connected to ground / the batteries the loose cable just has to hit the metal to produce big sparks and to get really warm. Like red hot.