It is possible only if you dropped the Silver Cross before your meeting with Rebecca and the King.
Assuming you did that, then just before you enter the room where you encounter them a second time, go through your inventory, and FILL UP EVERY SLOT BUT 1.
The best way to do this is arrows or similar items that can be divided up as much as necessary. (You also want these to be trivial items that you don't mind getting rid of. Again, arrows and similar are good for this.)
After the Bane King's final dialog, Rebecca will appear. When you get the opportunity to respond to her, you say those three magic words that every girl wants to hear, and she gives you a diamond ring.
This fills up your one remaining item slot. Since she still has a key to give you, the game puts you into the NPC encounter mode, where you may trade and talk with an NPC. Use this to give Rebecca one of your useless items (one of those arrows) to free up another slot.
You will then have the opportunity to say those three little words to Rebecca again. Either she's a sucker for your charm, or she just has more diamond rings than she knows what to do with, so she gives you another ring.
Repeat as much as necessary, but for the last iteration, be sure that you get rid of TWO items, so she can also give you the key. (Otherwise, you'll be trapped in conversation with her forever.)
That's my recollection, although it's been a while. Let us know if it doesn't work.