I can borrow against house 1 to fund the new purchase. For tax purposes would it be valid to state my purpose as borrowing to enable keeping the first property? (and hence claim the interest as a tax deduction).
No. This is not possible and your argument is not logical either.
If you own property 1 outright then how would borrowing against it enable you to keep it?
You will be borrowing against one property to buy a new property. If the new property will be owner occupied then it will be a private expense and interest won't be deductible.