It's impossible to get the transmission in two gears at the same time (it's a constant mesh transmission - half the gears are splined to the transmission shafts and the other half freewheel), but if a freewheeling gear seizes to the shaft it's the same as being in two gears at the same time. The centrifugal clutch engages at about 2000 RPM and is in full lock at 2500 to 3000 rpm and is not adjustable, but the manual clutch is adjustable. If the clutch adjustment is too tight, is can release the clutch as the engine heats and most noticeable in the higher gears. You can test the release point of the manual clutch by pressing or lifting lightly on the shift lever. The shift lever releases the manual clutch to facilitate shifting. If you just barely press on the shift lever and the clutch releases, adjust the clutch release on the clutch cover for a later release. If you have to press the lever about half way through a shift to get the clutch to release it is about right. If it's about right at this time and you press on the lever to get it to release while holding the throttle wide open in high gear, then let off the shift lever and it continues to slip until you let off the throttle and perhaps slips again when you get back on the throttle, then the plates probably need to be replaced.