Annecy and Lyon in one day

No. And I think you need a minimum of two nights in either one of them to even begin to enjoy what they offer. We liked both locations a lot, but that was because we weren’t rushing through either one of them.

It’s about 2 hours (each way) between them, so yes you could do a day trip from one to the other. Could you do both and NOT spend the night in either? While technically possible I’m not seeing a compelling reason to bother with a few hours in each place more time driving or train vs doing when both are such amazing destinations.

I interpreted your question to be asking if you could tour both cities in one short day. That’s why I said no, Physically, yes you can go between them by train in the same day. Hope that clarifies.

We spent two nights in Lyon last fall and barely scratched the surface -- basically we did the traboules and the basilica and Roman ruins. Another day would have been nice.

I suspect Annecy is similar. We are doing 4 nights there in a couple of weeks and hope to spend time in the villages around the lake. Fear that it may already be overrun with tourists which has happened to Paris -- thinks are in full swing again

Your other thread seems to say that you would be starting from Geneva. As a day trip from Geneva, Annecy makes sense (by bus or by train), Lyon does not.