I tried adding a new part slot in the vehicle customization menu but it's not appearing anywhere in the menu
I do have a part. I was trying to duplicate the fuel tank to add another engine to the pessima Not yet. I don't usually have to do that.
Why do you need a new slot for a new fuel tank? Just make your new tank use the "pessima_fueltank" slot
Oh, that's A LOT more complex than that... Believe me you don't want to do it, a lot of struggle to get a twin engine working, including writing a bunch of LUA code for the changed powertrain system. Not worth it, stick to using existing mods that add twin engines.