Actually, I have not played with looping the same clip twice
which is what it sounds like you are asking. However, unless it has
a strange ending, you should be able to put two copies on the timeline
with a fade out on the first, a fade in on the second and by moving the
overlap a bit, make it fairly tough to hear the cross over (where you
enter the delay between audio tracks, if you use a negative number
like "-2", it will overlap the two tracks by two seconds. You can also
do it in Audacity which might give a better result since you can
zoom in and work the issue of getting the beats lined up so it sounds
right. That would be my approach.
One other thing you might consider is the Digital Juice "Stack Tracks" -
they are what they call "layered music" - each piece is actually made up
of 4 to 6 or 7 "layers" (drums, organ, guitar etc) and you can turn them
on or off as you want with their free software - you can have the same
piece twice but once with the guitar and once without for example which
gives you the "same but different". They do have an interesting special
on (just got email from them today) - if you buy one of their "combo 5 packs"
for example ($149), you can get 5 additional stack tracks volumes for $1 each.
mikey