The banks are going to have to find imaginative ways of dealing with this problem. Maybe they are already doing so - though I have not seen any evidence of it.
As I see it, the bank could ( and should) 'move' the negative equity from house A to house B provided that the figures are such that the bank would be no worse off.
In practice, this would mean that the OP would have to fund all transaction costs from savings, rather than from increased borrowings. However, if OP can put together a transaction where he\she goes from a negative equity of €50k on property A to a negative equity of less than €50k on house B, then it may well be in the Bank's interest to support this.
Obviously, it would not be in the interest of any other bank to wade into this mess - only the existing lender.