Mortgage interest is only allowed where the money was borrowed to buy an investment property. You are borrowing to buy your home, so you will not be able to claim tax relief on the interest.
It makes no difference from a tax point of view, whether the mortgage is secured on your home or your investment property.
Likewise if you borrow money secured on your home, to buy an investment property, that interest is allowed for tax purposes.
It raises an interesting issue. Should you sell your existing home and buy your new home for cash? Then borrow €280,000 from the bank to buy an investment property. With stamp duty reduced to 1%, this is probably a sensible strategy.
But the bigger question is whether you should have a property investment of €300,000 when you already have a property worth €800,000. It seems to me that you have too many eggs in the property basket.