Finish your back routine--ALL of it--FIRST dude!
In this case, the only thing I could see you doing
after curls would be hyperextensions.
Alternating between, say, rows and curls is going
to kill the iron you're moving in the former (and
in pulldowns and/or chins).
In almost every standard back movement--and
by back I assume you mean lats for the most
part--the biceps are your weak link. The lats
are the biggest muscles in your upper body,
and have probably the greatest potential for
strength. (Doing strict pulldowns with 400 lbs.
is a good bit different than doing even paused
bench presses with the same weight: compare
the distance you'd have to move both.)
So, make the weak link even weaker by wearing
it out, and you'll have an even *harder* time
nailing your lats. You won't just be taking your biceps
out of the movement...you'll be taking your *only*
chance to work your lats out of the picture, too.
It'd also hurt your deadlift, if you do them...imagine trying to pull four plates for reps after you just took a grueling set of curls to failure. It ain't gonna happen. You need that damn grip!
Besides, your back routine should be a lot longer
than your biceps workout. You'd quickly run out
of things to superset. Consider that you've got
rows of maybe two types, some kind of pulldown
or chins, shrugs, and hypers to do, presumably each for a
few sets...if you did only what your biceps needed, you
*might* do two exercises and some forearm curls. That's
not enough to spread around.