bayleaf, sympathy. I have no experience of working in the public sector, or of having career which is so highly in demand (you lucky,lucky, thing!)but I can really relate to your feeling that this acting head has a secret agenda.
My last head of department pushed through decisions about our department which culminated in two of us being made redundant. At the time I felt powerless to stop the train of events and it was clear to me, and to others in our company, that he had an agenda.
Before I went on maternity leave with my second son, my appraisals had been very good, I had a good reputation wiht otheres in the company and my head of department had strongly urged me to apply for an upcoming promotion, promising he'd support me. The minute I told him I was pregnant, his attitude towards me changed. He made it clear tome and to my colleagues he no longer wanted to mentor me. When I returned from maternity leave and compassionate leave following the death of my mother, he started to criticise my performance. Over the next year he tried (unsuccessfully) to get me to leave by making life very diffucult for me especially, but also for the rest of the team. Many times he was reprimanded about his bad managment style and lack of support for our team. When we had notice of redundancies (due to some loss of business)I felt it was a given fact that he would choose me over a direct rival for the remaining job.
It was only a month or so after I left that I heard that this other person was his lover, they had both left their spouses and had just bought a house together. He must have been seeing her during the redundancy process and for who knows how long before. I'm not saying my boss blatently saw me out of a job, but I can't help thinking that he would have been hard pressed to have made his own lover redundant. Tellingly, he didn't reveal his out of hours relationship to anyone until the redundancy decisions had been made.
You are not in this situation I know, but from my experience I'd say you should trust your hunch about a secret agenda. I'm not sure where you go from there, bayleaf, but I think it would pay to be very wary and keep your ear to the ground.
It does sound, though, that the acting head just might not want the two of you to 'gang up'. You and your collegue sound like you are highly regarded at the school and both already have head of department experience. Perhaps she fears your combined power? do you think she is seeking to initiate policies in your department that you would both disagree with? Or, as others have said, do you think it's the fear that you will both take maternity leave? are the other teachers who she has targeted likely to take maternity leave too?
Also, you say that there is nothing to be gained from talking to her. Yet these actions of hers are controversial when you look at how other staff have been selected at your school. Do you think she is hoping to provoke you and actually expects you to tackle her?
Agree you and your colleague need trade union help - hope things become clearer. I do feel you don't know all the facts at the moment. It's a rotten situation to be in.