Excellent explanation Many D's. Back when I was a kid there where lots of great built up systems that used dual risers and the was my specialty.
When sizing a dual riser the horizonial line of course should handle the full load. The smaller riser should be sized for the load when the compressor is fully unloaded.
Example: If you have a 20 ton compressor that can be unloaded down to 25% (5 Ton) Then the horizonal line would be 21/8 the risers would be 11/8 and 13/8. 11/8 would be good for 5 ton and 1 3/8 would be good for 15 ton. Air conditioning.