dos eno
 

is it possible to use a dos eno double freewheel along with a chain Tensioner so i wouldn't have to move the wheel when i switched cogs?

Because a chain tensioner (I'm thinking of the Surly Singleator or one similar) have a lateral adjustment for the jockey pulley to ensure precise alignment, I would say no. I think if you could adapt a spring-loaded model to accept a urethane roller as found on the Rennen Rollenlager, you might be able to keep the chain tensioned and not have to worry about derailing the chain. The Rollenlage itself won't work for your purposes as it is not spring loaded and would require you to loosen the adjusting bolt when switching between cogs.

The idea behind the dos is to use two chain rings with the same tooth difference. When switching between small cog/big ring and big cog/small ring, the effective wheelbase moves only slightly.

Right, but if the setup is already run with a tensioner, it would need to be retained. Also, maintaining the same chain length does not take into account the lateral movement of the chain. The roller (or pulleys) on the tensioner would need to move over the same amount as the chain.

Try the Paul Melvin.

-Rob.