This was discussed in http://www.mobileread.com/forums/showthread.php?t=159972. What I said there was:
This would probably be up to me, given that I wrote much of the custom column definition and edit code. Unfortunately for this request, I don't have any personal need for this feature and am thus not motivated to do the work. If anyone else wants to take it on, then the widgets are built in gui2.custom_column_widgets.py. Some interface would need to be added to specify and store the order of the widgets, something would need to be done to keep things working in the face of changes to the columns, and the difficulties of arbitrary placement of comments fields in the two-column layout would need to be resolved.
Potential workaround for edit metadata single: use the standard 3-tab window and set the tweak metadata_single_use_2_cols_for_custom_fields=True. This might help, even if you have so many custom fields that they won't all fit on the two-column layout. Edit metadata bulk always uses the single-column layout.
I will leave this ticket open in case Kovid wants to comment or assign it to someone else.