Simple D&D 4e rules question

For some reason the part of the PHB clarifying this is not appearing to me as I peruse the thing. I’m sure it’s right in front of my eyes.

Every few levels, you get to add two attribute points. Can you add both points to a single attribute, or must they be to two separate attributes?

Haven’t played that edition, but from a quick perusal of the “Gaining Levels” section (p27):

(bolding mine)

That’s correct; the bumps at 4th, 8th, 14th, 18th, etc., have to be applied to two different stats.

At 11th and 21st level, you add 1 point to all 6 stats.