Sancerre, I would:
1. read the port of choice into a single BYTE size variable.
2. AND port read variable with $F0 and place into separate variable to separate the upper nibble. (4 bits)
3. AND port read variable with $0F and place into separate variable to separate the lowerr nibble. (4 bits)
There you have it. I am assuming you are talking about BCD when you are talking about decimal?