Two identical ones for sure not. They are usually made for a specific F-ratio of the incoming beam, and deliver a faster beam on output. Therefore, the next one has a 'wrong' input.
Also, they are usually made to also 'fix' things like field curvature. If the first one corrects that, the second one would try to fix it, but instead start introducing it in the opposite direction.
Finally, I'd have not the foggiest idea about proper back spacing. For one reducer this is a given value. You can place your camera in proper distance to the last of the two reducers. But what distance to put between the two?
In short - if you have them, play with it. But don't expect (too) much.