quite right tying 2 +12 volt lines will just increase amperage and using a split rail to make a 24v difference (-12 and + 12) can be done, but yes you will need to regulate it as it will float and you dont want to send negative voltage into any logic system that is not prepared for it , and I dont know how a switching supply will act over time

It is not the worst idea I have heard this week in electronics, but I would read up on it so you dont go cooking your new toy