Knotting isn't necessary. I thought it was and finally decided that all these experienced knitters and crocheters just might know what they're talking about. They do. If you're not confident of the knitting 2 strands together, go ahead and weave it in some before cutting it, with acrylic yarn it is a good idea. Leave yourself enough of a tail that if it starts coming loose you can redo it, but you're not likely to need to. It's possible to weave in ends as you go, links below. Currently I've been playing around with this method and like it for the new end as well as the old end, I weave in the new end before I start knitting with the new yarn.
LinksReally Clear: Weaving in Ends as You Go
I've not done 2 ends together, just one at a time.
Weaving in Ends as you go, No yarn needle required!
I don't know why she ties a knot, it's not needed. She says you can't do it with purls, it can be done, I do it, I can't explain it...at least not yet.