Particle hair gets messed up when I join objects

I have the hair nice and combed but when I join the head mesh with another, the hair gets totally screwed up.

I created a plane for my eyelashes, which I’m trying to duplicate for the other eye. To be able to mirror them perfectly to the other eye, I need to first make them a separate object so that I can apply the mirror modifier only to them, and not to the whole head (unless there’s another way of mirroring). When I try to join the mirrored eyelashes back with the rest of the head, the hair gets completely screwed up.