Single idler tensioners like the Singleator won't take up much chain so your rings would have to be pretty similar in size. IIRC the limit is about 5-6t difference. The Melvin would wrap up more chain.

But IMHO it seems silly to have both a FD and a tensioner. You could be better off with just an older rear derailleur like a Huret Svelto which is no bulkier than a Melvin and has enough adjustment that you can limit travel to two cogs. I do this to add gears to Sturmey Archer hubs but you can just run two cogs on a freehub or FW with spacers.


Svelto, Sturmey 4 speed FM hub, KMC 1/8" chain, 18t Nexus cog and 24t HyperGlide cog.

2 cog setups shift great, much better than front shifts. Even with crappy cogs and chains from the '70s. With Hyperglide cogs and modern chain, shifting is amazing, even on the 22-34 I did for the wife. Chainline is great compared to a normal derailleur set-up.

Another possibility is that Sturmey is coming out with a modern 2 speed kickback hub w/coaster brake.