What would happen?
You may well do. Check this with Andrew Humphrey, who knows a lot more about such things than I do, but as with IELTS certificates, there generally needs to be complete agreement between the number on your identification document (usually a passport) and all the other documentation, so that it's clearly not fraudulent. It may be sufficient for you to submit the two passports together, but I wouldn't count on it. Talk to Andrew, and check to see what UKCISA and the UKVI say. Ultimately, it's the UKVI you need to keep happy. Good luck!