Just to understand - do you have 2 properties region1 and region2 or you have 2 values region1 and region2 in the single property?
If you have 2 properties, then:
=EPMDimensionOverride("000";"COMPANY_CODE";C1&"="&B2)
Where
B2 - page axis cell containing Region member ID
C1:
=IF(A1="CAT1";"REGION1";"REGION2")
A1 - page axis member for CATEGORY
You can also create property in CATEGORY dimension like REGION and read it in C1
Vadim