I have a Mountain Muffler string for a 58" recurve. Can I make it work for a 58" longbow?

the brace height should not be right for the two.
a recurve bow generaly have a  1 to 2 inches higher brace heigt than a longbow.

It probably won't work, longbows have a slightly longer string than a recurve at the same respected bow lengths (58" for example).  The longbow string would be around 55", and the recurve around 54" for starters.  The problem is if you unwind the string to fit the longbow, you may take out to many twists if it's a flemish string causing it to come apart.  Conversely, if you try and twist the string too much to fit a bow, it can produce negative effects such as loss of speed, damaging of the string, and overall bow performance and quietness ususally goes out the window.  Just my observations anyway.

Thanks guys. I was also told that the loop sizes weren't compatible. Thought maybe there was a magic string trick someone knew that I could borrow    Now I have 2 recurve strings thatwon't fit any of my bows    :(

owlbait,
have you tried the string on your LB?
Not? Then do so and see what happens ...!

If your bows were build and designated according (that's the point!) to AMO specifications - hence AMO 58" bows - then any AMO 58" string (which would be 55" long) should (an other point!) fit them. This was one reason to develope the standards in 1968 - after all ...
Problem is, far to many have (had) no idea how to apply these in correct manner to their bows, strings and/or don't follow the standards at all - which is okay of course if they don't pretend to have ...

With all this confusion there is only one way left: Try it and see what happens! Good luck!
Falk