What happens if you mix 1 oz of 20 volume and 1oz of 10 volume developer to hair color?

Mixing 1 oz of 20 volume developer with 1 oz of 10 volume developer results in an effective volume developer somewhere between 10 and 20 volumes. This blend may provide slightly more lift than a 10 volume developer alone but less lift than a 20 volume developer. It is important to consider the hair's condition and desired outcome before using this mixture.