Exhaust should open first followed quickly by the intake.
An easy way to remember this is to think about how a 4 stroke engine works. As the piston comes up on the stroke that the valves open it will exhaust the burnt gasses on the way up (exhaust opens) when it gets to the top of the stroke the exhaust valve closes and the intake valve opens so that as the piston moves back down it will draw new fuel into the cylinder. The next time the piston comes up both valves stay closed so that the fuel compresses, when the piston gets to the top this time the fuel will burn forcing the piston back down, then up, exhaust, intake, compress, burn, ect....
PS: don't even try to mess with the gearing in the back, you will get the timing hopelessly out of wack very easily.