My department is splitting. Should I be on one side or get a joint appointment? I am a tenure track assistant professor in the "Department of X and Y" which is a single department for now.  Though my PhD is in X only, I now teach classes in both areas.  My research is mostly in the field of X, and my tenure committee consists of researchers in X.

These two areas have a significant salary imbalance.  The starting salary of assistant professor in Y is close to the salary of full professors in X.

This department is about to be split into "Department of X" and "Department of Y", and I sense that this may not be a very happy divorce.  I am now approached by the future Dept. of Y to be involved.  I can see 3 options:


Stay in Dept. of X and forget about Y.
Stay in Dept. of X and get a courtesy appointment from Dept. of Y.
Get an joint appointment.
Move to Dept. of Y completely


How should I evaluate my options?

My feeling, so far, is that option (1) should be the safe option.  I may loose most of my students, but likely stay out of trouble. Option (2) means I teach for both departments, which may not make sense, as I will be getting my salary from Dept. of X (much lower) and do half of the job in Dept. Y.  My teaching and service in Dept. Y will also not contribute to my tenure case.  Option (3) makes more sense financially and can potentially broaden my research area, but it may be impossible to keep two department chairs happy (especially when there are conflicts between the two).

Are there other factors I should be considering?

Update: Option (4), moving to Dept. of Y completely, is not an offer that is currently on the table. It is unlikely to ever be there, as I don't have a PhD in Y.
As in comments, for sure avoid getting yourself in the cross-hairs of political fights, as a tenure-track person.

Next, indeed, having mixed allegiances is not a good thing when you come up for tenure.

Possibly "being interdisciplinary" is a good thing, depending entirely on departmental politics. I've seen it go both ways (in my U.S. R1 univ): evidence of not being a real mathematician [sic], or, alternatively, being one of a new breed of mathematician.