"Am I changing POVs if I'm describing what two or more characters are sensing". Not necessarily. If you are writing in what is awkwardly called "omniscient POV" then you can can tell what any number of characters are sensing without changing POV.
Also, don't confuse POV with narrative first person. Just because a character is narrating does not mean that they can only narrate what that character sees in the moment. That is just one particular and very restrictive narrative mode, which we might call first person stream of consciousness.
" I know it's not okay to jump from one POV to another." You are misinformed. It is perfectly acceptable to jump from one POV to another. Good writers do it all the time. It is, of course, possible to do it in a way that is jarring, and that sometimes gets described as head hopping. But just because a technique can sometimes be done badly is not an argument that it should not be done at all. Indeed, since every technique can be done badly, not doing anything that can be done badly would mean not doing anything at all.
Also, there is nothing wrong with explaining what a character is sensing. Often this is the only way to let the reader know what is going on. The phrase "show don't tell" can be very misleading in this regard. In prose all you have are words and all you can do with words it tell.
The question is, which things do you tell directly, and which things do you tell indirectly, allowing the reader to work some things out for themselves. So, do you say, "Tom was nervous" (direct) or "Tom shuffled his feet" (indirect) or "Tom shuffled his feet nervously" (both)? The answer is, it depends on what impression you want the reader to come away with. The impressions that people form for themselves can be more powerful so telling them that Tom shuffled his feet may make them feel his nervousness more acutely.
But this is still telling. It is telling something physical so that the reader may intuit something emotional. But you can't do this for everything. You would end up in a pattern of infinite regression and you would bore the socks off the reader in the process.
She saw Jenny's nostrils flare, as if they itched
This is not preferable to telling us that Jenny's nostrils itched, if that is the information that matters to the reader. It might be appropriate if what you wanted to get across to the reader was that the Nina is obsessively concerned with the state of Jenny's nostrils. But if you are doing it simply so we know that Jenny's nose itched, then what you are actually doing is giving us a false impression of where Nina's attention is focussed.
Fiction is all about focus. Obsessively trying to remain in one POV or trying to give all information obliquely dissipates focus rather than creating it. But if you can focus the reader's attention where it is supposed to be at all times, variation of POV and the various methods of exposition will pass entirely unnoticed.