I'm a bit confused.
There's a difference between the XML contents and the promoted property.
You can set the promoted property to merge the contents. But that affects only the column (promoted property) and leaves the XML data intact.
I'm guessing that you could merge the data within the form, and use a specific delimiter (which doesn't occurr in the data). So let's say you use a delimiter like *.
Then, when the data is queried, you could (using code, I think) break up that data back into a repeating structure, based on the *.
It's a bit complex; we could try doing this using the DelimitedList command in qRules, but it should be possible using code as well. Good luck!