Also can someone explain this part to me?

Lesser rod of extend spell x 2+ - 6,000 gp. False life, greater magic weapon, mage armor, unseen servant, etc. Tag the whole party. 3rd level spells like GMW might wait a level or two. Get all up 24 hours, so right now 14x2=28 hours. Can also use pearl of power on 1st level spells like mage armor.

How can you stack extend?