it is alledgedly never optimum to mix and match batteries with different type ratings or different age, but i have done it in my RV's. And the batteries have seemingly lasted a 'lifetime'. Maybe luck of the draw.
They will very probably charge at different rates though, the two different batteries, so IF you are going to go this route, charge them seperately, not in parallel. One may need a longer slower charge. They will also discharge at different rates too, so one will inevitably be getting worn out quicker then the other.
But, like i said, when i 'forgot' to tell my dealer to DELETE the 12 volt battery from one of my trailers years ago and got 'ripped off' for the price of it, ... well you use what you got for one reason or another. And it lasted the 3 years i abused it by letting it run down to a zero charge and also run dry. I wanted an excuse to go out and buy a series of deep-6 batteries and never really got it.
I now have the Deep-6 batteries on my current RV, and i still have that deep-12 in my garage that i use emergencies.
One 12 volt is going to run down quick while boondocking. I personally would put the two of them together .... and when they give up their life eventually, go buy 2 or 4 deep-6 trojans.
So i guess my answer to your question of "can I , should I??" ..... well theory frowns on it, but my experience says you'll do it since you now own it anyway.
F350 PSD 4x4 SRW, BanksBrake, FOX res shocks, MagHytec, DP tuner, JohnWood tranny;
'63 hog; ChrisCraft Launch VP8.1 ;
3 hound dogs