When adding small diameter (2#) line to the backing line , I tie a knot using a three turn improved clincher knot to each line for a small knot . It just has to hold the lines together under the primary line . With 4# try a two wrap .
When spooling new line , I hand wrap four or five wraps over the knot at the top of the spool .
Never had any issues .
On my 1000 size reels , I’ll usually do halfway with my backing line , that usually gives me about 125-150 yards of 2# primary line .