Bernard Cornwell probably gives you a lower bound on how many you can get away with, and I'm fairly sure it's more than two....
http://en.wikipedia.org/wiki/The_Saxon_Stories

(Of course he's writing a series where conflict with the Vikings is a central theme of the whole thing, so two could still possibly be too many in a different book. But if there are good plot reasons for both, and the descriptions are repetitive, I'm sure two would be fine.)