At 50% wear I would replace both, 1 so you don't get pull left to right, 50% isn't great but not bad so it is has served you.by the time it is worn out your other tire ( the 1 new one you haven't bought yet) will be 50% worn out and you will be in the same place you would be now with a mismatch set of tires. And Peace of mind. But if flat tires are common for you that may sway your decision. Just my thoughts
