Thanks.

If they get hot while the switch is off, there’s got to be a short somewhere. You might want to try a different switch. If you have a meter or a circuit tester, check continuity across the poles of the switch when it’s in each position. It could be that the switch itself is odd or that the poles you connected your wires to are always connected for some reason. Could be that there’s a solder bridge across them or even one or two small strands of wire touching. You want to make sure there’s no connection between the positive and negative battery wires. Try it with one fan and one battery on the switch and test.

If you’re wiring the fans in series, there could be issues if they’re wired incorrectly. If the fans are wired in series, then the batteries must be wired in series as well. I prefer one battery per fan, myself, so if one battery drains faster I don’t lose both fans.

You can’t wire two fans to one battery in any case. the current draw will drain the single battery very quickly and possibly cause some heat.

This may sound silly, but double check that you know which is the positive connector and which is negative. on 9V batteries I find myself questioning that from time to time and it isn’t always marked clearly on the battery. If you’re using someone else’s wiring harnesses, you might double check those too.

I’ve always used 9V batteries to run small 12V fans with no issues. They just run a little slower. I expect you’ll be able to sort it out. Let me know if you have any other questions after you’re done testing. I hope this helps.