What Interest relief rate are revenue likely to allow on Loan

I am looking at two investment properties and have been approved in principle for a loan. The bank will be treating both as commercial mortgages and charging the commercial rate. I will only be buying one of them all going well. One of them is all commercial so I will be getting allowed to write off all the Interest as an expense.
The other property is a shop unit with two overhead apartments. It is being sold as a unit. My question is if I purchase this one with the commercial mortgage will I be allowed the full write off of the Interest or will I have to put a value on the overhead apartments and a separate value on the shop unit and then allocate the mortgage proportionately to each type of property.

If I do it this way I will probably only be allowed 75% of the interest on the apartments and 100% of the interest on the shop unit.
Would this be correct?.

That's right.

Thanks Joe_90. Stamp duty is 2% on commercial and 1% on residential under 1m

If they were valued separately ie commercial/residential could I pay commercial stamp duty @ 2% and 1% on the residential without creating separate deeds?.