Separate circuit may be easier since "electrical isn't (your) strong point".
It starts with which heating pads you pick and what comes with them. They could have a timer to shut themselves off after some period of time but if not (or possibly either way), I'd want them to not be able to run when the vehicle isn't running (or at least key in the accessory position) as that's a significant draw on the battery.
I'd pick a circuit that's only live with key in run or accessory position, run that to the coil of a relay rated above the current of the heater element (at 14.4V, not just 12V), run the other pin of the relay coil to a switch, and run the other contact of the switch to a chassis (ground) point.
Next run whatever wire comes with the heating elements, or if you have to add your own consider what current it needs to carry to pick a suitable wire gauge (or lower). I'd guess you could run two seat heaters at 20 amps or less so you could get away with using 12ga or lower wire.
Run that from the battery with a fuse at the battery (fuse value calculated as done with the wiring to be just a bit above what amps current you expect the two heaters to draw, and a relay rated for more margin, say 30A DC), to the relay power pin. Run the other relay power pin to the heating element, then the other heating element contact to chassis ground.
I would make sure the wire is copper, not that copper clad aluminum junk common today on Amazon, ebay, etc (or else use an even lower gauge to make up for that) and would not use a generic Chinesium grade relay, sticking to a major brand instead.
That wire running from the battery will split to a duplicate of what is described above for the 2nd relay and switch for the 2nd heating element unless you only want one control to turn both on and off simultaneously.
Where to put the switch? I suppose you might be able to put it on the side of the seat where the controls are there, whether you use a switch included with a kit or have to buy a different one for size or looks or whatever. Current rating won't matter for the switch since it is only switching the tiny amount of current the relay coil uses, so it can be small and inexpensive.
You could do it without a relay and just use a larger more expensive switch, but significant DC current is hard on switches, you're better off with a relay.
