Mixing speaker impedance

I'm wondering if I can connect an 8ohm Celestion 8 inch speaker cab to the "out" of my 16ohm TM112 cab from a TM Deluxe 20?

I have the 8 inch cab laying around and would love some low-volume stereo sound from the two speakers connected to my amp. I haven't been able to Google up an answer that makes sense to me.

Not recommended I'm afraid Dangercow.  The out on your TM112 is a parallel connection to the speaker inside.  Hooking up an 8ohm impedance speaker to the 16ohms that you have inside it will give an overall 5.33ohms and that's too low to work with from your 8-16ohm H&K output.

And, not wishing to be too nitpicky but you won't get stereo sound out of that setup anyway.  You are sending the same signal from a single amp to both speakers, it's really binaural and won't give any sense of space or time delay, only a fixed central mono image (assuming the two speakers were balanced volume wise which these two wouldn't be anyway).  You would need to have separate amps driving each speaker with either different settings or a different effects selection to each before you got any sense of a true stereo effect.

Gotcha, thank you for the explanation, that all makes sense now.