having worked on a tandem eccentric last week, it seems that the OPs original idea my work. get the chain as tight as possible and there is a certain amount of slack permissable in tandems, perhaps 1/2 to 3/4 inch as it wears in.

i have never tried this, but i am able to pull the timing chain gears off my tandem when it is fully tight and then slide them back on. maybe if your timing chain slacks too much, use a half line for adjustment.

again: don't know if this would work, but would seem too.