For special cases like the one you mentioned where linking between two donations meant for one device was not made, please contact us in mail.
What makes you think so? We're already at ~120 devices ordered and down payment paid, and counting. For very obvious reasons we cannot concatenate devices into one, initial donations were meant to count as one device ordered already, and we acted accordingly. GDC subtracted wages from the original donations which we re-added to the voucher value since donors paid for the development which will benefit their device - if we concatenate multiple devices into one, we would concatenate that "value add" which I don't know how to fairly calculate. Of course you're free to find new customers to use your voucher, any time. It stays valid infinitely. However keep in mind that supply might be limited eventually, if we completed sourcing of some risk parts and can't get new ones. Or you keep your voucher and wait until end of project, when we actually can calculate how much to deduct for R&D before we could refund the voucher's remaining value.

cheers
jOERG

update 2015-06-01: 153 and counting