Vaccine 'Pick and Mix'?

Although it is early days (the very first approved vaccination, in the UK, was carried out today using the Pfiser/BioNTech preparation), people are already talking of a possible new trial, to commence in January 2021 (https://www.theguardian.com/world/2020/dec/08/covid-mixed-vaccine-trial-likely-to-begin-in-uk-next-month). The Pfiser vaccine is the only one, that has currently passed through all the regulatory hurdles for safety. It seems very likely, however, that the Moderna (another mRNA technology-produced vaccine) the Oxford/AstraZeneca (a more traditionally-produced vaccine) preparations will both be soon approved for UK use. The vaccines vary in their efficacies (i.e. their ability to prevent an infection with Sars-CoV-2) but the duration of protection is uncertain. All are given in 2 doses, several days apart. The Scientists want to establish (and they would need to do this as a separate trial, unrelated to the initial safety testing of each compound) whether giving different vaccines in the 2 injections improves the efficacy of treatment. Efficacy could be measured in terms of the duration of protection, as well as simple initial avoidance of an infection. It seems worth trying, as people would be exposed to 2 different antigens (foreign proteins) that could trigger an immune response. That might be better than exposing them to the same compound twice.