Mirror Modifier at compined object has diffrent axis

[Peetzii]

Hey I want to connect vertexes of 2 parts of 1 object which were mirrored.
I modelled every part on its own and joint them but something went wrong with their axis I think…

I thought of snapping them which worked on some parts but the axis makes it almost all the vertexes impossible.
I dont even know how to name the problem with the axis… how can I solve this or is there another way to attach the vertexes of those to parts to each other?

[rigoletto]

I guess your origin is messed up or did you use an mirror object?
Set the origin/mirror object to the center of the open face of the half head. Or simply to an vertex on the outer rim of the open face.

[Peetzii]

yes I mirrored it but I can’t connect the vertexes this way.
can I switch it somehow to the same side for both parts?

Update: I solved it, thanks a lot!