thats what I do. I have 2 55 gal drums up in the attic 3/4" line down to a float valve in the rinse tank, and a 3/4" overflow line into the rinse tank. When that overflows it just goes into auto bay. but I use as much as possible.
Check out this thread.
http://www.autocareforum.com/showthread.php?7720-Reject-Water/page2&highlight=reject
The link takes you to the second page in the thread. Click to page one to see the illustration.
If your IBA tank holds nothing but rinse water, it is much simpler to not use it at all and use only the reject tank. The dumping from this tank to that tank requires more plumbing, valves, and whatnot, resulting in more things to maintain. Don't get hung-up on using the original tank unless it is absolutely necessary.
My wash just doesn't do the amount of cars that some of your washes do, I'm thinking 55 gallon drums in the attic will be my best bet.
I plumbed my reject line to a 250 gal. tote. I used a Little Giant pump inside tote to pump reject into touchfree wash HP pump tank.
The Little Giant pump has a low cutoff float switch and a second float inside the HP pump feed tank.
works for waxman