Hi Amy
Sometimes dogs might get a bit jealous of each other when both want the same thing. Females especially can be very "bitchy" and become quite competitive over stuff and unfortunately you might be that stuff that your Yorkie is getting jealous over. Your Yorkie has found a good way of putting her point across and is telling your Schipperke that you belong to her and to stay away so that’s exactly what your other dog is doing.
To help make sure that your Yorkie is not seeing you are a resource to defend and get jealous over, it is important that you seek help from a qualified companion animal behaviourist. In the UK you can look on the APBC website www.apbc.org.uk, they also have a great reference section for recommended books on the subject.
Be aware of people telling you this is all about "dominance” it’s more likely to be all about "wants" and how strong and brave your Yorkie feels.
It is important that you don't spoil her by giving her stuff on demand; this might be attention, love, food, toys. Instead from now on you decide when she can have nice things not her. Then before she gets anything she has to sit and wait first and then give it to her when she is sitting nicely and not snapping or trying to grab at it. You don’t have to stop loving, playing and fussing her just do it when you say and not her. Be in control of fun things.
This should be once under supervision from your behaviourist since sometimes dogs can become quite frustrated and snappy when something they expect is taken from them, so in this case, since bitch aggression, can be very serious I would have a good chat with a qualified and recommended behaviourist.
I hope this helps and good luck
Rosie Barclay