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^ this.
what you want is impossible to do well. Chain tension needs all parts to stay in their place. Chain tensioners don't work on fixed gears because they cannot handle the forces on the chain being applied 'in reverse' i.e. when skidding/slowing down.
As you probably know, a slack chain can jump off the cog/chainring. When this happens it easily gets caught between the cog and the chain stay, resulting in a crash/very sudden halt.
Two-speed hub might be the way to go :)
edit: some more thoughts on this here. You could go for the 'ghost ring' option but when you're also gonna shift the chain and thus change the chainline and have that awkward moment when its between rings, I just wouldn't...
Smolders
The only reason I can see it not working is that chain length is going to be different between the rings, so you'll have no chain tension on one ring unless you move the wheel in the dropouts.
A chain tensioner doesn't work with fixed, only single speed.