Here's a sitch that happened this summer.

Last play of half. A throws an INT. B player is tackled via the facemask.

After the play is over, B commits a UR.

Ruling? (Fed, NCAA, etc...) I'll post the Canadian ruling later.

I give, what's a UR? I'll asusme it's equivalent to an Unsportsmanlike act.

I think both codes will agree on this.
The interception will stand.
Assuming team-B wants to accept the penalty against Team-A, it will be tacked on to the end of team-B's run. Being tackled by the mask is most assuredly the 15 yard version of the face mask foul.

Extend the period if accepted.

Since the play ended, the dead ball UC foul by the intercepting team will also be enforced as this would not be combined with the face mask foul.

UR = unnecessary roughness

Doesn't change your enforcement any, except that as a personal foul, it doesn't carry the same potential ejection consequences.

And, of course, in the hugely unlikely event that B declines the penalty, the half is over and the dead-ball PF against B will still be enforced: on the second-half kickoff.

So in Canada, if a UR happens after the last play of a quarter, it is applied in the next quarter. However, since the first UR occured during the play, the non-offending team can choose to apply that penalty to extended the period for one more play. Once that happens, the period is no longer over, and the second UR can only be applied in the quarter in question. So then, we have two URs - both applied at either point ball dead or at a point of application of the first UR. Either way, 15 yards and 15 yards will balance out and we have one play.

Another official and I are also bb officials and we termed this a "false dual foul". :D

(In Cdn fb, a dual is the same as a Fed bb double.)