Corporate Politics - Struggle Between Managers

Hello,

I have an issue with a support struggle in our office, so need some feedback from anyone to help me out. This is it: I started out in a position about 3.5 years ago to support/report to a VP of Mktg - about 3 yrs into it (6 months ago), we hired another person SVP/GM and started supporting him in a sharing capacity. The VP of Mktg left the company about 4 months ago, so my direct report/manager was/is the SVP/GM of the company (head of a business unit). We then most recently hired a new VP of Mktg about 1 month ago - I now will be supporting this person in a sharing capacity (he is in a different department and does not report to the SVP/GM). The dynamics have changed since I first started at the company, so now my essential duties are the same but have added other misc duties. The VP of Mktg that I am also supporting (not reporting to) is going around my manager's back and now wants to consume my support as if he were the only person. He is commanding more of my time and is not discussing it with my manager - he also sees my position in a different light and wants me to be more like a manager and direct his team on the flow of work. This is not what I was hired to do - I plan on discussing all of this to my reporting manager soon. Am I being a bit sensitive or can you only have 1 primary manager and many secondary managers? I've never experienced this before, but it is giving me some serious issues. Any help from you would be most appreciative. Thanks!