A "Split King" is a bed with two Twin XL mattresses in a king-sized bed frame. QuickZip offers packs for split king which include fitted sheets for the two twin XL's and covers (in the Classic and Euro packs) in king size:
Split King Starter packs
Basic: 2 Twin XL Bases, 2 Twin XL Zip Sheets (Equivalent of 1 fitted sheet for each mattress)
Classic: 2 Twin XL Bases, 2 Twin XL Zip Sheets, 1 King Flat Sheet, 2 King Pillowcases (1 “set” of sheets)
Premium: 2 Twin XL Bases, 4 Twin XL Zip Sheets (Equivalent of 2 fitted sheets for each mattress)
Euro: 2 Twin XL Bases, 2 Twin XL Zip Sheets, 1 King New-Way Duvet Cover and 2 King Pillowcases
And if you are ordering Add-on Zip Sheets for your set, the Split King Add-on Zip sheet contains two TwinXL size zip sheets.