In Boyle’s Law, can an answer be determined if two values are not stated?

Not really. Not if all you have is the one relationship. P1*V1=P2*V2 has four unknowns. This requires four simultaneous equations (relationships that state new facts and not just a re-write of that same relationship). Normally, when we say a "known", that is an equation: P1=7 kbar. So, you would need three knowns (three such equations) to be able to solve the fourth.

In essence, sometimes the information is there but you have not seen how it provides an additional equation that can be used to solve the starting problem. If you only know two values, all that you can know is a ratio that applies to the other two unknown values. You have a line that represents all possible solutions, not a single point. You require one more relationship that defines something in addition to that linear relationship.

For example :
What happens to the pressure of a gas if its volume of is doubled at constant temperature ?

In the above question, both the values of P₁ and V₂ are not given.

At constant T: PV = P'V'

Given V' = 2V
so P' = PV/2V
P' = P/2
The pressure will half

You have to assume a pair of missing values are constant. Of course they must be the same variable. (Often this is obvious from the context).