For anyone reading this - https://github.com/ajshort/silverstripe-itemsetfield is a great alternative
in addition to validation handling we added some other features to the module yesterday.
1) counting the items in both columns and show the number under each column
2) checkbox "Select All" on both sides
3) search filter above the columns that filters both cloumns while typing
We didnt test them very extensive but at a first look they seem to work fine.
If you like i can give you the code so that you can integrate it into your next release.
Itemsetfield quickly got the job done. Thank you ajshort.
My question is if there is any way to change $default_options per page type. I haven't looked into changing ItemSetFields default options.
I'd like to make this Sortable, and Turn off the Picked items in the search.
For example, The new config accepts options in the 4th argument as an array
$fields->addFieldToTab('Root.Content.Testimonials', new HasManyPickerField(
'Pick a Testimonial',
'Sortable' => true,
'ShowPickedInSearch' => false