Chris - but wouldn't this relationship be stored somewhere in SAP? I just feel you don't need to manipulate select options at all, although I'm not sure what data you'll be getting exactly. You might want to post what tables you're planning to read - if those are standard ones (those that don't start with Y or Z) then we would know them too, it's not a secret.
For example, for table KNA1 (customer master) we could have the selection options for account number (KUNNR) and country (LAND1). The users could fill in either one of them or both or none.
Then there would be a SELECT statement like this (just a very simple example):
SELECT KUNNR NAME1
INTO TABLE itab_customers
FROM KNA1
WHERE KUNNR IN SO_KUNNR
AND LAND1 IN SO_LAND1.
If, for example, the users only select 'US' for the country (LAND1), the program does not need to go and fill in SO_KUNNR with the customer numbers that fit that criteria. SELECT statement takes care of that. If both selection options are left blank then SELECT would just read the whole KNA1 table (blank select option translates to "include all", not to "equals blank" - this part can be confusing for beginners).
Again, not sure what you're doing exactly further in the program but I feel you might be just misunderstanding how selection options work.
