Dear Peter,
Unfortunately, I don't know of any easy way to have invoice and credit note numbers share a counter without modifying the extension's code.
Best regards,
Reinhold
PS: If you want to modify the plugin code to have the two number types share a counter, I would start at app/code/community/OpenTools/Ordernumbers/Model/Ordernumber.php, functions getCounterValueIncremented and loadNumberCounter. The easiest, hackish way would be to simply do something like:
if ($nrtype=="creditmemo") $nrtype="invoice";
at the beginning of these functions. This should make all credit note numbers use the invoice counter, too. Please note that I have not tested this at all. Your PHP developer would need to make sure that this really works properly in your shop...