[Joe Johnson]

The idea is to move the gears closer together so that they are always turning with the input gear.

Then put an idler sprocket on the same shaft as the input shaft (now the shaft marked with the green cross).

Then pivot the both output shafts on a lever that also pivots about the input shaft (it is outlined roughly in a T shape in the image attached).

Then when you drive this link, you engage one sprocket or the other with the chain. (The sprockets are those round black circle things)

You can have the air cylinder either be strong enough to take the load you are pulling on the chain -- in which case you sort of get a chain tensioner built in for free. Or you can add a drive link between the aircylinder and the sprocket pivoting gig - in this case, you can make the whole thing non-backdriveable and then tension in the chain cannot push the mechanism out of engagement (I like this solution better... ...but time is limited so I will have to discuss it another time if there is interest).

The chain routing is shown for the mechanism in high gear (the thickish blue line).

I hope this helps.

Joe J.