You can have two vouchers if you received one last year and one this year. And it will say that you have two vouchers. It is very clear.
That's correct. I have two - one earned for 2009 spend and another for 2010. And even though you're allowed to redeem two vouchers on the same booking, to obtain award flights for four passengers for the miles-price of two, do note that you cannot currently redeem both of your certificates for one booking on BA.com. The website only allows you to choose one or the other of your two, not both.