That's my point, combine both accounts and make one reservation!
It doesn't work.BTDT. We have five nights in Rome. The first four had to be under one Hilton account to get the 5th free night. It's not really a free night. It's 20% off each night which is okay if you're not using weekend certs. I had 2 nights booked with awards (160k points), 2 weekend certs, and wanted to add the 5th free night. CSR had to cancel my previous reservations. I went from having positive points in my account to negative and needing more points to get the 5th night. Very frustrating. I had the email confirmation of my point change with adding a free night but could not get anywhere.
Having said all that, if there's a work around, someone here will know it. Whether they share it or not is another question.