I run BIAB and Reaper together as this is my preferred method. I know there are fans of Realband but I find Reaper to be much more stable than Realband. I did miss the ability to regenerate specific bars in Realband for the real tracks but I now prefer to edit in Reaper. That being said here is my work flow with both. Beware it's lengthy, probably too long but if it helps then it's worth it, right?

I create the basic song format in BIAB as one continuous chorus. Once I'm satisfied including the ending I will drag and drop each individual track as a WAVE file either into a reaper project or into a source folder to add later. The beauty of BIAB and real tracks is that you can regenerate as many times as I want and save each rendered track as a wave. I usually generate multiple takes of the same instrument (sometimes up to eight) because I will usually edit them in Reaper. I also take the time to render other instruments because you never know. If you keep the song file open in BIAB while working in Reaper you can also test other instruments on the fly while working on the project in Reaper.

As far as editing, some may find my method a bit anal but it works for me and you may find a better way. Here's a sample on how I edit a guitar track. I first render the guitar track from BIAB with the DI option checked if it's available. I find you get a much more uniform sound if you add an amp simulator onto the Reaper track. You also have the option of using Amplitube in BIAB on a DI track and you can render from that. You will have more flexibility if you do it in Reaper though as your stuck with the rendered sound from BIAB.

I create several new empty tracks in Reaper and import each rendered track of the guitar track from BIAB on to each track. I then select and highlight all the tracks,right click to bring up the menu. There is an option to implode all selected tracks to takes on one track. Reaper will consolidate all the tracks on to one track as separate takes. You can now split the takes all at once however you want. I split every measure (sometimes each beat). Takes some time to do but the result is that when you press play you can audition each take separately selecting and building each piece as you go. It gives you much more control and creativity than regenerating in Realband and waiting for that to take place. You can truly come up with some unique tracks and it keeps you in a creative groove. It works great for drums,bass and piano too.

Then when your happy render the finished track and mute the original (keep the original!!!!). The rest of the work is done in Reaper. I will say this though do try to learn how BIAB works and how to use the bar settings. You can save yourself a lot of edit time by letting BIAB do some of the heavy lifting especially trying get holds, shots, rests and endings. This is an area I struggle with and need to spend more time learning myself.

This is but one of many ways to work with both programs together. Hopefully you can get some great ideas that will work for you.