The yellow ones takes a flesh sample when equipped, that’s tested for disease in a lab. The sample is tiny.

The white one is an electronic tag, able to be scanned by a vet (I assume some other officials too) for relevant information on the animal, like their breed, date of birth, etc.

Having two is good in case one of them is pulled off of the ear, so it can be replaced. Also they’re easier to read that way, seeing as there’s a tag on both sides of the head. All cattle in the EU have to have 2 tags (if they lose one it’s fine, just should be replaced pretty quickly)