They're the same. It's just a second dose as a booster.

I believe only Sputnik vaccine has different first and second dose, but their vaccine is of different category (it belongs to the same as AstraZeneca and Johnson and Johnson). The reason is that these vaccine use a vector (adenovirus) and there's a risk that body will develop antibodies for the vector and the second dose might not be as effective.

"Some vaccines require two doses because the immune response to the first dose is rather weak. The second dose helps to better reinforce this immune response." - I would have to think over time that could be optimized somehow to just require one w/ ML and test results, etc.