This issue has cropped up sporadically for us since the DayZ ArmA 2 mod. We started playing two client updates ago, and it was sporadic (meaning we could sometimes connect to some servers at the same time). With the next-to-last client update it was fixed entirely (we were able to connect to any server at the same time), but with the last client update it broke completely (we are unable to connect to any server at the same time).
When the issue was sporadic, it didn't seem to be tied to certain servers; sometimes one server would work, then the next day the same server wouldn't let us both in at the same time.