Two gaskets?

[Twostrokepower!!!]

Would two head gasketson my kx85 increase overrev?

[VintageBlueSmoke]

No.

Short answer I know but you are not asking the right question. Changing the thickness of the BASE GASKET will increase/decrease the squish area of the cylinder. This is where compression happens. Increasing compression, generally increases horsepower. Not enough squish area and nothing happens except a lot of throw away parts (hole in the piston, a broken, rod, destroyed cases). Too much and either the cylinder bottoms out, breaks off, and wastes your main bearings, (just see parenthesis above) or there is just not enough compression to cause the violent explosion that forces the rotation of the crank for the next cycle.

If you have a paper HEAD GASKET (not an o-ring like many modern water-cooled motocrossers these days), you can accomplish much the same thing there.

Basically, there is a sweet spot. Engine builders and tuners find that spot using a dyno and then duplicate it. With stock porting, a stock base gasket is usually best. Changing it will also cause you to change your jetting and possibly your fuel. More compression will require a slower burning (higher octane) fuel.

Make sense?

[beaner]

Stacking head gaskets will decrease compression and power.

[MyckMcClung]

I agree with what these guys said, it is true yo uwill lose compression. I run two high compression base gaskets on my KX250 to take a little of the violent hit out of it. Major improvement in the rideabilty department for $3. I ride tight eastern woods, if I were an MX guy I'd run 1 high comp base gasket.
 It's not always about the most horse power. An easier to manage power band is often more benifitial than an over the top drag motor, simply due to the rider being able to hang onto the bike for longer periods of time.
 I know several guys that try to tap as much power as they can get out of thier bikes, but can't use it 95% of the time, you'd be better off spending some money on setting up your suspension properly for your weight and riding style. It will truly make much more of a difference in your speed and consistancy than trying to squeeze 1 or 2 HP out of your motor

[Toadmeister00]

I'm going to disagree somewhat and say that stacking base gaskets may increase the top end and overrev. Adding a gasket will raise the ports and drop compression slightly, which can give you a gain up top. To do it right you'd have to have the head modded to tighten up the squish again.

In my supermoto/roadrace bikes I run very tight squish but lower compression.