Hi M.,
I am a step-mom to my husband's 12 year old. My husband and I have been married for 6 years and have a 3 year old. His mom has also remarried and has a two year old. Here is our schedule of shared custody:
We have him every other week (change day is Monday). On the weeks that we don't have him, my husband has a "date night" with him on Thursday evening (so he doesn't have to go a whole week without contact). He also has nightly phone calls with the parent he is not with. Things have evolved a bit since the original agreement. I am a stay at home mom (his mom works), so I have him for school days off, summers, and before and after school (I have replaced daycare). We also have him every Sunday morning for church (we consistently go to the church he has always known, whereas his mother has church shopped a lot). All four parents usually go to school functions, birthday parties, and trick or treating on Halloween (with mutual friends). It is by no means easy! Some applaud us for our efforts to put differences aside for the child. It is becoming increasingly difficult! He is spending so much time with us now that he feels more comfortable here and doesn't want to leave. He also is becoming more aware of parenting and having a hard time with the way his mother lets his little sister get into his stuff, scream and tantrum and bite. They are fighting more and he calls us to complain and she calls us for advice. I suggested they all go to therapy to work things out (and leave us out of it!) and now she is not speaking to us....It's a mess. I am the one who spends the most time with him, but I don't have legal say!
So...I think you should work with a lawyer and mediator. It really is easier on kids to have one home, but it comes at the expense of one parent, who will never have the same or as close a relationship with that parent. It really depends a lot on your husbands motives. It's also worth mentioning that if the kids spend more time with the girlfriend and become close to her (I know, hard for you) then it will really be a blow to them if that relationship ends. I would try to incorporate something about only introducing kids to "serious" relationships" and have them be "friends".
Good luck! None of it is easy!