We know about the protection stats for one vac and how much is added by the second. That's tested reported fact from the manufacturers verified by independent scrutiny. We don't know how long protection lasts for with one vaccine or two....nobody does. Its already been said that Covid vaccination may need to be offered annually like flu vaccination
As I see it, worst case from the knowledge that we have, is that the second vaccine behaves like a first vaccine and reducing immunity, if it does reduce, goes back up to what it is from a first vac.
I have said before on here that a friend of mine's husband who is a vaccine trials volunteer has been told that if he is offered the vac, he should contact the trials hub to be told what he was given (which vaccine or placebo) BUT if he is offered it 12 weeks or more from the date he had the research vac then he need not worry but should just accept whatever vaccine he is offered.
I genuinely have no idea what "we" should be worried about. Personally I would much rather see the vaccine shared as widely as possible as quickly as possible.