Im about to install my exhaust into a true dual.
I got racing beat road race header, and from there its 2 1/4 piping all the way back to two Dynomax welded performance mufflers.

My friend gave me his Ultraflow X mufflers that he bought for his 5.0 mustang.
This muffler is uses an xpipe in its design and from what I gather from searching is that any form of x or h pipes will hinder a true dual exhaust for a rotary.
http://www.dynomax.com/products/ufx.asp

I was about to use it to quiet down the exhaust, but Im still going for just a straight pipe with the muffler at the end. If it is too loud I can just cut the middle section and put in another muffler with flanges incase I want to change it.

 

you should be able to i wouldnt use a muffler with fiberglass for a premuffler but if you have a few feet of staight pipe to cut out before the y it would work after the y it would be a bit more work