Lend me your brains

but you can't do this for both combinations at the same time. So you'll have to find two magic-gear ratios that will allow you to run the wheel in the same positon, or one chain will be slack.

Yeah I think you are right, unless I choose my ratios carefully I might have one chain a little slacker that the other.

But I will work the maths out and, if needed use a half link, to keep the chains fairly taught.