Outside string tied/wrapped with serving or string material itself. I've also conveniently used plastic ties as mentioned above, but found that those abrade the string over time and they just don’t look as good (if that matters to you).
Between the string works, but I don't like when people put a huge “glump” of Cat Whisker between the string strands, as that to me, is counterintuitive for performance. If you do go between the string, you’ll likely want to tie them as well, as mentioned above.
Several ways to do it, you’ll find your preference.
Kris