Active / Standby pair

Hi Folks, It's been a while since updating the sw on our LTMs so just wanted to confirm my understanding of what ver can work together as a pair. We currently have a pair of LTMs ver 11.2.1 FX 6 and I would like to go to HF14. So i should be able to do 1 of the 2 and leave for a few days before doing the other one. There should be no issues with having to different HF's in an active/standby configuration? What about having 1 as 11.6.0 and the other as 11.2.1? from memory i don't think this will work but just wanted to confirm.

Thanks.