Using iMessage from iPad created 2 message threads

Hi,

My girlfriend and I are both using iOS 5 on our iPhones. Up until a couple of days ago everything was going fine - SMS and iMessage messages were appearing alongside each other in the Messages app. The other day, however, I picked up my iPad and sent her an iMessage from there, and it created a second user/stream in her Messages app. Now, whenever I send her an iMessage from my phone, it appears in the new/separated thread but when I send an SMS it goes into the original thread which contains all our other messages (including all the iMessages sent before my iPad messed things up).

Also, when she opens the original thread, she is only able to send me regular SMS/MMS, and in the new thread she's only able to send me iMessages. Is there any way to get the two threads combined in her phone?

Incidentally, this only happens on her phone. In the Messages app on my own iPhone, everything is still combined within the one thread and from there I'm able to send as SMS or iMessage.

Both threads on her phone are titled correctly with my name, and tapping the "Contact >" button at the top of the message thread displays the exact same contact card for me.

I've tried going into her contacts to ensure my phone number and registered email address are both contained within the same contact - and they are. I've even tried removing my contact details entirely from her phone and re-importing them, but that didn't work either.

Does anyone have any ideas how I can solve this issue and merge the two threads (or message streams if you prefer) please? In the meantime, I've had to turn iMessage off entirely as it's too hard to follow the thread of a conversation when my messages can be appearing in any one of two places.

Thanks