My question somehow similar to this one, but here what i'm asking here is how to properly ask to HR time to consider their offer, i got an appointment for today in the evening from company A (which i haven't confirmed to go yet), i'm waiting for company B final offer (which is my first option), they could take at least 2 day more to give a final answer.
State clearly that you need to consider and how much time you expect that you need. This is no negative thing per se and actually quite usual. I would ask for a little more time than the 2 days you mention above, this is better than having to postpone because company B does not come around.
If you are approaching your deadline, you can also call company B and tell them that you need an answer because you have another opportunity.
You can be open about having different offers, but should not use it as argument for wage-negotiations. When you need time to decide between concurrent offers, the official reason is always personal growth/more interesting field of work.
Just make sure you communicate open and honestly and meet the deadlines you set for yourself. That shows you are trustworthy and also self confident early on. Remember, you are the asset in this transaction here!
Note that of course there is always a small risk that the company will find someone better-fitting in the time you need for considering.