Maybe
I think you would have to move other furniture also. Best I remember there is a nightstand between the two double beds. (I just checked the floorplans and there is a nightstand with the lamps between the 2 beds). And I'm not sure how close together you could push the two beds. You may be able to get the beds close, but, I wouldn't think it would be like a single bed.
As far as room selection goes, you should be able to select the building you would like to be in upon check in. Because they check in on all days of the week, you just have to go with what is available at the time. We have 2 weeks at PBSB and although we are allowed to request certain buildings we don't always get what we request due to availability.
Edit: I have to disagree with geekette that they would move out the two double beds and replace with one (larger) bed. Although they can be accommodating, I just can't see them doing that.