Will you own 2 properties? You can only claim tax relief at source (TRS) on your PPR (principle private residence). You can only ever have one PPR.
You can claim TRS on the jointly owned property, but you will have to stop claiming it on the first property.
You can claim a deduction for interest paid against your tax bill if you are renting out the first property.
For TRS purposes, you are classed as a first time buyer for 7 years from the date of your initial claim.
If you rent out your first property within 5 years of buying it, you may be liable for a stamp duty clawback.