An alternative to the idler is a "ghost" chain ring: A chain ring a bit larger than the ones on your sync chain, that floats at mid-chain engaging both the top and bottom chain runs. It may take some experimentation, but as a first cut, spread the two runs apart, and measure the verticle distance to the outside of the chain, then look for a chain ring that measures about that diameter. You can also use two slightly smaller ghost rings if that is what you find. There is virtually no stress on the ghost rings, and minimal chain engagement, so worn out (for normal use) rings will probably work fine and you might get these cheap or free at a bike shop.