Performing sequential and asynchronous custom validation

[PerceptControl]

I stuck in situation where I need to do serial db queries on validation stage. For example:

{
   Validate(aExistInDb)
   fieldA: string
   
   Validate(bExistInDb)
   fieldB: string
}

Let's assume validators "aExistInDb" and "bExistInDb" both are async and execute some select queries from db

In my situation field b relates on a. It means that I want to check fieldB only when I sure that "a" field exist and correct.
I expected that "bExistInDb" validation will run only when "aExistInDb" ends. But now both starts immediately.

What shall I do then?