My son and fiancee purchased a home in 2016. Both names are on the deed/mortgage. Can the mortgage interest deduction be split in any ratio, or must it be 50/50?

[gjmarx]

One makes more money than the other, so it may be advantageous for that person to claim the whole deduction, if legal.  Since they are not yet married, they are filing Single.

[MichaelL1]

It can be split other than 50/50.  For example if one pays 70% of it, they would claim 70%.  For interest to be deductible in must be paid, and you must be liable for it   

You are both on the mortgage, so are both liable.  

Just make sure you are both in agreement each year, so together you are not claiming more than that is on the Mortgage interest statement (1098).