A question was asked recently in the context of a specific pattern. The pattern contained chains that faced opposite directions in an edging. It would be an easy "switch shuttle" if one were to use 2 shuttles. But she wanted to know if it could be done with one shuttle and a ball?
Yes! Of course, the easiest way is to do a shoe lace trick (SLT) or overhand tie, but you know I avoid it as much as possible, preferring other options such as Reposition, Twist Work, Reverse Join, and/or Reverse Stitch, whichever is most effective at the time. (See https://tipsaroundthehome.blogspot.com/2020/07/lock-join-plus.html)
Here's a quick sampler I tatted to show one way of How to Avoid Using 2 Shuttles - using direct tatting.
3. Reverse work after completing the segment. This brings the shuttle to the top ...
4. .... and tat the 1st ring of next repeat. It looks like a thrown/floating ring.
5. Rest of the motif is again worked as alternate chains and rings, with RW between each element. Repeat the direct tatted chain and continue for desired length.
My answer is that it may be a matter of preference, sometimes mood! It can also be a means to reduce the number of tails to hide especially if it's going to be a looooong edging. Of course, one will need to either have 2 balls of the same shade (in order to reload the shuttle) or preload several shuttles before using the remainder of the ball for the 2nd thread.
One other scenario is when the rings are worked as mock rings (MR) or as self-closing mock rings (SCMR), a single shuttle and ball can work well.
Personally, I do not mind hiding ends, and 2 shuttles are more portable and manageable (a shuttle won't unravel if it slips off, unlike ball of thread). 2 shuttles also provide a lot of flexibility and freedom.
As I always say, there's no absolute right or wrong in tatting. Choose what you feel comfortable with, or what the pattern demands, or how the pattern can be adapted to colour(s) and techniques. My point is merely Be Aware that there are several paths to the same destination, and with a bit of thinking ahead and adaptation you can travel any road!