Mix and match subs?

I see no real value in having them match. Obviously. https://systems.audiogon.com/systems/8367 In fact, the main benefit of multiple subs is more small modes adds up to smoother bass. So to the extent they are different this creates even more variation in response, which for low bass is good. That is why mine were made 2 ported, 2 sealed, and they work beautifully together with the one ported isobaric Talon Roc sub. That is 5 subs of 3 different types. Just running out of new things to say about how misplaced an idea it is to talk about the importance of matching, integrating, timing, etc. There is a grain of truth to it. But it is so much less important than sheer numbers of subs as to pale in significance.

Then the question becomes should I go with the x12 to get the extra extension, or a second x10 for easier integration as many of you have suggested?

Depends on what kind of music you listen to and how loud. If your current X10 is loud enough on its own, you will be fine adding a second X10. If you need the deeper extension, then you should sell the X10 and go with two X12's. Integration happens at the crossover point, so mis-matched subs won't matter, but the benefit of multiple subs only happens in the frequency range that both subs share. So if you have music that goes lower than the X10 can reproduce, then you will not get the benefit of having two subs in that range.

No, it does not work that way. One of the many things people realize when they get a DBA is how much extreme low end information there is even on recordings with zero low bass that can be heard as such. Similar to the way extreme ultrasonic high end cannot be heard as such, yet does create a greater sense of realism on all recordings, extending all the way down into the low bass region.

These simplistic views of sound and what we can hear are really, uh, simplistic.

Generally speaking, it doesn't matter (unless one of the subs is really terrible).  I recommend you take a look at Rythmik's line of subs for music.  SVS makes good subs also, you won't go wrong either way.  More is better when it comes to subs, unless you're in a really small room.

I’d go with the bigger second sub.  As mentioned above, matching exactly isn’t as much of an issue with subs, but that added bass down to 20Hz or so could add some real magic in an expanded soundstage and better imaging along with better depth.  Again, I’d try a PB1000 Pro because it’s literally free to demo at home and the value is very high.  Best of luck in whatever you decide, but I think you’re really gonna enjoy having two subs no matter what you choose.