So if you register your AR receiver as an SBR, then add the under barrel shotgun, I don't know if you would need a second stamp for having now created a second SBS or not?
If you mount an 870 with a barrel length less than 18", and an OAL of less than 26" to an AR, the 870 needs to be registered as a SBS.
The only time you'd have to register the AR as a SBR is if the AR itself has a bbl less than 16", or an OAL of less than 26".
If your rifle had a 16" bbl, and the attached shotgun were 12.5" (the shortest you can make an 870 without requiring MAJOR gunsmithing), you would only have to pay one $200 tax stamp, for the short-barreled shotgun.
If your rifle had an 11.5" bbl, and the 870 had a 12.5" bbl, you'd have to pay two $200 taxes ($400 total). One for the SBR, and one for the SBS.
Of course, you could throw another $200 stamp into your collection, and toss a suppressor onto the end of your AR. If you were going to do that, you'd want to make sure you had a top-of-the-line suppressor, one that would function properly with a bbl as short as 11.5". You'd really have to run that short of a rifle bbl, because if it were any longer, the suppressor would be susceptible to being struck as the shotgun started patterning.
Now, that would be the ULTIMATE tacticool AR. An 11.5" bbl, suppressor, rail-mounted Rem 870 SBS @ 6 o'clock, some form of optics @12 o'clock, a Surefire light @ 3, and a IR pointer @ 9. Hmmmmmmmmmmmm........