I agree that Samsung Duos phones don't support rSAP on the second SIM. I used a S4 mini Duos previously and I have now a new J5 2016 Duos.
The Android RSAP app cancelled the network connection on the SIM 2 in the S4 mini, when SIM 1 was connected in rSAP. This brought the advantage that calls on SIM 2 were forwarded to SIM 1 because SIM 2 was offline (Settings in S4 mini).
However, the downside of this phone was, that the second SIM was not reactivated an the end of the rSAP connection.
I've just tested the J5 (rooted with TWRP, Android RSAP app) for a few minutes so far. It seems to me that similar to the S6 Duos described in the previous post, the second SIM stays active in the phone.
It would be nice, if it could be configured in the app that the second SIM goes offline, when the first one is in rSAP mode and both turnes on again, when the rSAP connection is closed.
Edit 2017-01-03: I've realized this now with Automate from Llamalabs. All together, this provides me now perfect functionality of Audi, Samsung and Dual SIM.