does anyone else know if you can do this ? for instance i have a best buy mastercard by hsbc and a regular hsbc mastercard?
Yep. I'm doing it right now, with exactly that combination.
I called and talked to Customer Service and asked them to combine the cards. I guess that confused the rep, as she put me on hold for a couple of minutes. When she came back she said she was submitting the request to combine the cards, that there was no guarantee that they would do it, and I should check back in a few weeks.
I think that was around a month ago. I didn't bother to call back, and never heard anything from them, so I assumed it wasn't going to happen. But yesterday I got a letter in the mail from them, with a form they wanted me to fill out in order to complete the combining of the cards. I just mailed it today, so we'll see if they actually do it.