Consider changing your negative holder sheets to the ones that hold three strips of four negatives. Then, when you decide to print one of the two "inner negatives" large, you can cut that strip of four into two strips of two, and store them at opposite ends of the same "channel".
Printfile's product number is 120-3HB:
https://www.printfile.com/120-3hb_25.aspx
They work well for 6x7 (nine negatives per page unfortunately), 6x8 negatives and for 6x4.5, when those come from cameras that give you 15 exposures per roll.