There's an Idea posted for the Mac version of Quicken suggesting to allow reordering of split lines in register transactions.
See here for reference: Allow reordering of split entries (38 Legacy Votes +4 Merged)
I think, the same applies to the Windows version of Quicken.
It would certainly be helpful if one could move a Split line entry up or down in the dialog or somehow reorder the sequence of existing split lines. Drag and drop, as suggested for the Mac version, would certainly be a plus.
If you are reading this and would like to see this enhancement in a future version of Quicken, please click the VOTE button on this webpage.