Re: Merc 99 225 question
If it were mine, I'd skip the kit. All you need is 2 14 gauge, or 1 12 gauge wires from the second regulator red wires to your second battery. Then be sure there's a 12 gauge or better bond between the negative terminals of the 2 batteries. If you have 2 trolling batteries, the 12/24 v switch in the system will parallel them so they both charge when on the engine.
I replace the bullet connectors on the yellow and red wires with soldered and insulated splices anyway. They are a source of trouble, as they push the limit on the connectors. Solderering it makes it a little more trouble to change something out, but it also lowers the probability of having to change something out.
hope it helps
John