I'm a fan of having 2 separate chain loops, one for front one for rear, it gives a lot of engagement on all the sprockets, and provides redundancy in the event of losing a chain. We had another robot run on top of us in 2005, and bend our front axles enough that one of our chains came off. But as long as we were flat on the ground (and not tipping forward

like it liked to so so much) control was not hindered much at all.
If you have a metal idler sprocket, then you still have 4 sprockets for each side, so any weight savings from having the single loop should be minimum. We used sliding axle mounting blocks for tensioning, each one with at least 6 socket head cap screws to hold it in place in the frame rail