Not at the expense of lowering the player count on one of the servers. Each one serves different player bases due to location; so one group would probably be against it. I would love it there were a third server with the lower player count and mixed game modes. But, as you said, if we're only scraping by to pay for two, adding a third would probably not work. There are times that both servers are full, so I think there would be a demand for it.
Its kind of surprising to me that there aren't more donations given the amount of players, obviously everyone is living in a different situation and it isn't always viable; but i see CoD4 servers that are basically dead (6v6 peak times) that have donations paid through until April of next year already.
I guess I'm lucky I'm in a decent situation, but I don't mind throwing in $15 or $20 a month if serious discussion around this develops; its at least a consistent monthly donation that can be relied upon. I've bounced around the last few years as servers I played on regularly have all closed (CoD4 mostly) so i don't mind chipping in where others might not be able to.