Dave is exactly correct. In some cases, knowing that they are likely to lose a prime candidate will motivate a company to move. I would also suggest that you explain to Company A that you would very much like to work with them, but that you have another offer (as Dave says, say nothing more) and you would like to know where you stand with Company A, as your career advancement requires that you proceed with one of the two companies.
I have personally seen this work. My daughter (in health care, not in the sciences) wanted to work for one hospital, and had positive feedback from them. She then received an offer, with a time limit, from a second hospital. She communicated with the first hospital, telling them that they were her first choice but she had to make a decision. Lo and behold, she was their first choice as well, and they moved heaven and earth to get an offer to her before her other deadline.
It also does not hurt to ask for a delay from company B. I had a similar situation with my first position. I was pretty sure that I wanted to work with the first company that I interviewed with, and they had essentially offered me a position at the interview, but I had 2 more interviews scheduled over the following week. I called the hiring manager at the first company and basically said "I like your company, and I'm pretty sure that this is where I want to work, but I have interviews scheduled at two other companies. I feel that I owe it to myself to go on these interviews to make certain that I am making the right choice going with you." His comment? "Go ahead, we can wait another week. I would rather have a happy and committed employee than someone wondering 'what if'." (Stayed with them, and the company that bought them, for 9 years.) Not exactly the same as your situation, but I'm sure that you can find a way to phrase your request for a delay.