Single HVAC System In a home with 2 units. Smells go right through duct-work!

I am currently house hacking with an upper/lower setup - I occupy the upper. The lower unit is a basement/guest suite. As I approach my move out strategy where I will be converting my upper unit into a STR, I often think of guest interaction amongst the two units. Specifically I think a lot about the smells that travel through the duct work between the units. We have a single HVAC system that heats/cools the entire home. Often I can smell scents from the lower unit that aren't pleasant (cooking, smoking, etc). One time I had a guest smoke and it went right up the ducts into my unit, it was awful.

I'm conscious about how that could potentially affect guests who will be staying in the upper unit in the future. I know I can't be the only one who has had this setup before, so I am just looking for thoughts, advice, experience, etc. Even if you haven't been in this kind of setup before, I am curious to hear your opinion. For us STR hosts, I know we are very conscientious about guest experience and I want to be proactive about doing things to ensure my business operates successfully.

Thanks!