My daughter has been the sole keeper for much of her club career, but not all of it. Of course, in HS it has been an adjustment because in her freshman year, they had two seniors, a junior and another really good freshman.
Like a lot of things in club soccer, it mostly depends on the coach. Her one year in club where she had a back-up, the coach was very young, inexperienced, and a bit of a used car salesman. Like 95% of most club coaches, he was a former field player and had zero grasp of the position. It was a bad experience for her because he reacted to mistakes rather than limitations (like in the article
@Grace T. posted in the other thread). If you're an NFL fan, you've heard the adage that the most popular player on any team is the back-up QB. Whenever the starter has a bad game (unless the starter is Manning/Brady level) everyone in the stands starts calling for the back-up. That's how it feels for keepers, too. The one in the game feels more pressure than usual to be perfect, and once you get pulled in a game after a bad mistake, it is pretty much over from a confidence standpoint.
I've met some coaches who preach always carrying 2 keepers at all times in the older age groups. I've met others who insist that you should only have one main keeper, and if they get injured, you call up another one from a younger age group or a lower team within the club. Incidentally, her favorite coach was also a former GK as a player, and he was adamant about not splitting time.
Another underrated factor is the relationship your child has with the other keeper (and the families). My daughter knows certain keepers that she'd love to split time with because she trains with them and has a great friendship with. She knows others that she has rivalries with, and I would think it would be a team chemistry breaker to have them on the same team. It's such a tough spot because it's not like the field players that often have to come in and out for rests, and can usually play a couple of different spots. They're used to coming and going, but a keeper has to really build up their rhythm, get in the flow of the game and often get lathered up to hit their stride. If you're constantly getting pulled at half-time or jumping into a heated game in the second half after sitting for 45 minutes, it is often the worst of both worlds for both keepers.