What I mean is you can take two different vaccines, but it won't result in two different antibody responses. If you take a less effective vaccine like the J&J, doubling up on the Pfizer vaccine isn't expected to 'upgrade' your antibody response, though it can still strengthen immune response. If spike proteins change the immune response may change, but in this situation all vaccine are based on the same spike protein.

The science is always developing, but mixing and matching vaccines tends to lead to more side effects according to at least one study. From what I remember personally in the beginning doctors were recommending people keep the same manufacturer but that isn't considered necessary anymore

https://www.verywellhealth.com/side-effects-mix-and-match-co...