First -- there is no such thing as a joint IVA. You will each have your own arrangement ... but, for administrative purposes they will probably have been made interlocking. They can be seperated, though IP's don't like to do this, and you can each carry on with your own arrangements and making your own payments --- it gets awkward for the IP if there are joint debts and / or you own a property together, but it is not insurmountable.
The alternative is that you still work together to carry on as now, leaving the iVAs connected and split the monthly payment between the two of you.
Depending on what happens with your living arrangements, a new I&E might have to be gone through to determine affordable payments.
Speak to your IP.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk