You didn’t mention which Mares Reg you are using, but most new regulators have at least 2 HP ports. Since you want to keep your current gauge set, this should leave with 1 open HP port. If your current regulator doesn’t have this additional port, it’s time to upgrade to a new1st stage. Adapters and splitters will only give more points for possible leaks or entanglements. Don’t try and cut corners with your life support equipment.