I haven't read back. You'll have to shoot me for that 
I am a mother and have been separated from my kids' father for nearly 6 years.
My ExH has the kids 3 nights a week. Friday night (so I can go out! )
Then he has them from Sunday teatime till he drops them off at school Tuesday morning. I have them the rest of the time.
So basically, we share care in terms of nights of the week although I have more evenings / meals / pick ups / other shizzle
Last year my ex moved in with his GF so I guess they have a SM (although they are not married and the kids have never referred to her as such, they just call her by her name and TBH I've never thought of her as their SM)
So, how do I feel about this? "as a mother" (because all mothers are the same and all think the same
I feel really, really relaxed about it. My kids have two happy, secure homes. And a routine that they know, love and rely on (in fact more of a routine than kids who live in a two parent home).
I use the time when they are away from me to socialise. To work (it allows me the chance to schedule in travel with my job). It allows their dad to spend quality time with his children (Remember that? Theyre his kids too!!)
AS for their SM, she is a lovely woman. I trust my ExH implicitly and I know he would NEVER chose woman who would not treat his kids like their own (hers are all grown up). I don't feeljealous or discombobulated. I am happy that my kids have another adult in their life (plus all of her family) to love and cherish them. It certainly doesn't mean I feel insecure or that they will love me less.
So OP I thinkYABVU to think all mothers are the same, and that a set up like this can never work. Because it patently does
Call us all sickeningly well-adjusted but it works brilliantly for us all. My ex