Nov 5- Doctrine & Covenants 128:15 & 18

I think it is pretty clear what is meant by the phrase that our dead can't be made perfect with us. They need us to preform vicarious work for them so that they can receive the works of salvation...But what on earth does it mean that we can't be made perfect without them. What can they possibly do for us? Well...a lot more than you may think!

Cook

https://www.churchofjesuschrist.org/study/general-conference/2014/04/roots-and-branches?lang=eng

Smith- Book Teachings of the Presidents of the Church: Joseph Smith pg. 475

Renlund

https://www.churchofjesuschrist.org/study/general-conference/2018/04/family-history-and-temple-work-sealing-and-healing?lang=eng