Could you upload your .rp file?
Take a look at this thread:
A basic approach would be to create a copy of your repeater contents in a dynamic panel to use as a temp or “fake” repeater object. Show this temp dp when dragging starts and allow it to be moved around. In its OnDragDrop, add a condition to test if it is over the other repeater. If true, update that repeater to add a row with the temp contents.