Just in case this is helpful for anyone else…
You can absolutely knit a bit on the body of the blanket and then switch to working the first edging before the body is complete. This allows you to check how much yarn the edging uses, so that you can use as much as possible in the body of the blanket - leaving just enough for the second edging.
AND, please don’t forget to go back and look at the June video tutorial on the knitted-on edging. The section with the yellow yarn is all geared towards this blanket, and shows the cast on as well as how to work the double joins.
Hope that’s helpful!