When you try to merge 2 different changes to one XML form description it is very probable that you run into problems. It seems like platform uses IDs, not items names internally, so if there are some objects with similar IDs at runtime platform present "Internal error" and when you try to edit form in Designer it is not editable.
I think it is very important to make it possible to merge form XMLs. With new IDE it will be very requested function.
p.s. right now we are using ordinary Designer
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