I don't quite understand your complaint here about Catalina sharing employment with Annika - the only skill they share is yoga, while Annika herself shares 2/3 with Riley. Also, having girls who share the same skill is crucial for having more than one or two guests - I use Catalina for yoga all the time because Annika is in the gym/sauna because Riley is in the restaurant because Amber is cleaning because Julia is busy, for example. If anything, I'm annoyed that Laura and Hana share the massage parlor because they also both have unique skills and juggling health/security plus massages can get complicated.
Correct. It's nice to have staff that can overlap in situations where one is busy with another task, the other can take her place to avoid a guest not having been satisfied. There've been times IRL for me (and I'm sure others will agree) that being in two places at once would be ideal if one could clone oneself. MwM isn't any different in that respect.
Early on in the game you had a difficult time satisfying two guests who wanted two different facilities simultaneously, but only one staff member to work the location (example; sauna and fitness). Since Riley got hired on in manor level 2, you only had one staff member who could work those facilities, which would lead to one guest or the other not being happy with their stay. That could sometimes be a problem, especially if you want the reputation level to increase.
Later on in the game, you had to made a choice to sacrifice security or healthcare in order to accommodate your guests. So you have to make a decision based on what's good for the manor.