If a couple split up during an interlocking / joint IVA, what happens to the joint debts, are they split or does the full amount get added to both IVA's. [?]
You would both continue to repay at the rate of your dividend until the debt is repaid or written off at the end of the iva. I expect whoever pays the greater dividend will have a moan such is the nature of break ups but when a portion of the debt is being written off anyway it's not worth worrying about who has more written off their half.
If the debt is paid off in full, the person who has paid more than 50% does have a claim against the other party under the law of equity - but it may be that they cannot be bothered to pursue this given the legal costs involved.