It would be a big help if you had some kind of simple drawing showing the constraints you have to work within and a few of the important dimensions so we can under stand where you are restricted. Its far better than trying to half guess someones interpretation in text.
You might want to consider another train of thought. Rather of thinking of two linear rails think of one, because of space and cost. You have to keep in mind these things are Zero play and designed as being a precision slide way for machine tools. It really is a case of 'if the cap fits'. if one will be adequate then up the rail size a bit and use one.
Kind regards,
Scott