Can I mismatch subs?

Subs in the same series will generally work well together. I think a second smaller sub will improve your system.

I'd save up and get another 113 or sell and match the 112's.If your system is 2 channel then smaller is beter.Less depth but and dynamics but more tonal accuracy.Placement and integration is a bitch no matter what and mixing and matching probably isn't a good idea.I think TBI had a great ides when they offered their subs powered or passive.Passive you could get their amp/x-over box and when you added the sub it was parallel and power to each box went up.Great idea that I wish more companies would have tried.I haven't done what Eric said worked but think it would be easier wot integrate same subs.That said I like multiple drive subs.
I used to sell Velodyne and latter after I left biz I became fan of multiple drivers to move large amounts of air.Light work the Vandertseen could fill in smaller Maggies.REL did same and speed and tonal accuracy is great with Quads etc.For HT and music Martin Logan has multiple driver speedy subs.I'd like to hear the Totem Thunder with 8" and two passive 8's.But you have a deep sub and might not want to give up depth.Still two Martin Logan Descents or Depths would be my choice especially if I was doing HT.
Cheers
Chazz

depends on what you are trying to do--where your xover will be --if you want the sub to xover in the 100-120hz range then the smaller sub will cover that better if you are going to use these in stereo and xover below 80 --I would go with 2--F113--I have my 2 F113 xover at 50hz in stereo ---which sounds much better than summed bass.

I had a F113 and an F112 in the same room at one point and I thought they worked together well. The F112 seemed to be a little faster to my finicky ears, but it was barely noticable.

I actually ended up selling the F113 and buying another F112 because i thought that the F112 did better with movies in my room and so they would match.