If you have two different Customer Account Numbers you will have to "realms" where you will then see two link spaces.  There is no way to merge two accounts.  What your options are at this point, is to contact support of either Quickbooks or ProSeries and ensure you get the order placed for whichever under ONE Customer Account Number.  Note that if you chose that, you will essentially abandon one of those link accounts but because there are two separate spaces with two sets of clients lists (even if they consist of the same people).  Most customers just chose to manage each one by renaming the account while within the settings of their link account.  Example ABC Company, Inc- QuickBooks and ABC Company, Inc.- ProSeries or something along those lines.