Yes you can have two different sizes. But no more then 2 teeth difference on most frames. It really depends on how long your rear drop-outs are, how much tire clearance your frame has and the size of the tire. For example most Surly frames have tons of clearance so it's easy to have 2 different gear ratios on the same frame...But if it's track bike or a road bike then you will have a problem sliding the wheel back or forward because of very tight clearance, the tires may rub the frame.