Two different 0% balance transfers on same card

Hi,
I googled but cannot find an answer to my specific question. I am 1 year in on a 35 month 0% balance transfer offer and I have been offered another 0% offer for 15 months on the same card.

How do the repayments work with two 0% offers of differing lengths on the same card? For example say I took the offer up and transferred £500. I repay at least £500 within 15 months. Is that one now cleared and there will be no interest until my original 35 months are up or how would I avoid any interest until the 35 month promotion ends?

Edit: Credit Card is Halifax