Hi jodo and a warm welcome to the forum
It will not be a great idea to leave an unsecured shortfall on this property and then enter into a mortgage with your partner for another one! The shortfall will not be able to be included in your IVA, and you will therefore have a "loose" creditor with no disposable income to be able to offer in settlement.
I also think that it is doubtful that you will be able to get a mortgage for more than the value of the property, to help clear off someone elses loan - especially as you are in an IVA.
Isn't it a better option for your partner to move into your property? And don't forget to update your IP about any change of circumstances if you start sharing household bills.
Is
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk