I don't see why not. They may read as exactly the same ability, but they have different names (and yes that's a thing apparently). So, yeah, they should stack.
That said, by the time you get this -- 13th level -- it's actually pretty useless for an illusion school specialist, because you're already getting 6 extra rounds after concentration. If you're not an illusion specialist, okay great, you go from 0 to 6 extra rounds. That's not horrible. Pretty meh for a second boon, but not horrible. But going from 6 to 12 extra rounds? That is almost completely pointless.
Also, by 13th level there aren't a lot of illusion spells where the duration after concentration is a big deal. IMS most of them are found at levels 1-4. Sure, even at high levels you'll still throw the occasional Major Image, but otherwise... I'm sure there are some higher level illusion spells where a few more rounds of duration might conceivably help, but I can't even think of any offhand. (Like, Mirage Arcana is concentration... plus hours/level. Another 30 seconds isn't going to make much difference.)
Now, Mahathallah's Obedience is amazing. It's two and a half, almost three feats for the price of one. It's absolutely the best obedience of any devil, including Asmodeus -- trust me on this, I went through every single one of them -- and it's probably one of the three or four best Obediences of any deity ever. But once you get past that, her boons range from Meh to lame to jaw-droppingly bad. (Seriously, look at those Sentinel boons. What the hell, man.) You could make a fun NPC with the Exalted boons I guess maybe, but... honestly, just take the Obedience, be a Lawful Evil illusionist specialist who is totally damned to Hell, do lots of drugs, and have fun with it.
TLDR: yes it should stack, but it's not worth bothering.
Doug M.
