I dunno - I think you could pull it off with a minimum of slack. Imagine if you had 43 x 15 and 42 x 16. When it was on 42 x 16, you would push the chain, while turning the crank, onto the 15-tooth cog, thus creating more slack, which you would then take up by pushing it onto the larger front ring. I think that so long as you did it in the right order, you could accomplish this within the normal amount of slack you want to keep your chain from binding up.

Of course, I have neither the resources nor the inclination to try this, since one speed is fine with me...