Hi,

You can use this module Account Invoice Merge by OCA which is available from v8 to v16.

Thanks

Today, this module merges customer invoices nicely: account_invoice_merge

https://apps.openerp.com/apps/modules/8.0/account_invoice_merge/ 

For merging two draft invoice you need to do some customization which prompt wizard and in wizard you can chose your draft invoice line and merge it. so this way it's possible to merge the two drat invoice

I have developed this thing for my customer requirement.