Why do climbing shoes have 2 loops?

The two loops are meant to help you pull your shoe over your heel. Sometimes it can be quite difficult to pull the shoe over your heel, especially for those who choose to downsize their climbing shoes, and having just one loop may just make it difficult to pull it over your heel, and could also shorten the lifespan of the loop since it alone will have all of the pulling tension. Eventually, it can tear.

Adding another loop allows the tension to be more spread out, rather than all of the tension being in one point. It also allows more area of the shoe's heel to be pulled, which helps make pulling the shoe over your heel easier.