Hi,
I believe it's the HR policy that determines whether an individual can receive one SL per month or two. Normally, they are accrued if there are no claims. If an employee can avail themselves of the same, then it gets deducted from their available quota, and after that, EL/PL can be utilized. Hence, it is 'leave with pay' until and unless your leave quota is balanced. After that, it is 'leave without pay'. In some organizations, advance leave can also be granted, but that depends purely on organizational policy or management decisions.
Your supervisor is the sanctioning authority; the deduction of types of leaves is something your HR manager can explain based on policy/quota availability.
Regards