When you use two taunt monsters it’s a 1/3 chance of bypassing and 1/3 for hitting each taunter. The “50%” is basically when you attack another monster you have the same chance to hit the taunter, so adding another increases it the same way.
The “67%” on auto-protect monsters is 2x chance to hit the protector compared to what you target. With two auto-protectors that’s 2/5 (40%) on each protector and 1/5 (20%) on what you target.
I’m giving you the numbers here because you probably think two taunters with “50%” means it’s 25% chance of hitting around them when it’s actually 33% chance.