there are two scenarios in this section.
If your Delivery Notes are not in 'Done' stage you can merge both with customization. Where the outlines are mentioning below:
1- Write some precaution checkings using default_get() function.
2- Write a function to merge DN where...
[Assume that the picking IDS are 11 and 12, You need to merge these and get finalized to ID 11.]
2.1 the picking ids. (11 & 12)
2.2 Take an ID out of them (It should not be the merged one.. ie 12)
2.3 get pack_operation_ids & move_ids of other picking. (pack ids & move_ids of 12)
2.4 Set its 'picking_id' as '11'
2.5 Unlink this picking ID (12)
2.6 Write 'min_date' as the date of the day.
2.7 Return true
Else if you are trying to merge the delivery orders that already in 'Done' stage,
It is difficult. try to get a dummy print out through defining a 'Dummy Delivery' report.
However, Which is not preferable because It will not give you a proper track.
Hope the hints are enough to help you.