Don't know your field. But if the market is hot, my personal oppinion is that it's worth to try company B while let company A wait for few more days. It's very common and reasonable that you ask for some more time to arrange your own business before on-board. If a employer has made a committment to hire somebody, it's very unlikely they will change their mind just because you may start a little later. It's not that much fun for the hiring manager/HR to go through the interview-hiring procedure, either.

If you did very well with company B, you may push them to make a quick decision, by letting them know that you have offers already. Acturally, in this way, you may even get a nicer bargin if you are really good.