Could you combine these and forego your own possibility of hiding behind your shield for an AoE and instead shield a companion within 5ft?
This would be the relevant part of Shield Master
If you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you can use your reaction to take no damage if you succeed on the saving throw, interposing your shield between yourself and the source of the effect.
Protection
When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. You must be wielding a shield.
I'm guessing not but the question got asked at my table.
Best Answer
No.
Each of these abilities provide a separate reaction and neither provides a way to combine them.
Oath of the Crown provides a similar effect
If your player is interested in playing a style where they focus on defending others you may be interested in the Oath of the Crown Paladin from Sword Coast Adventure's Guide. At 7th level they gain Divine Allegiance which lets them use their reaction to take the damage for another creature giving a similar effect to this combination.