I have this Carseat for Lacy in pink
http://www.gwlittle.com/prod/-HOT-PI...-Seat_530.aspx I have the regular size which is PLENTY big for just her - she's 7lbs. I can fit both her and Rylie in there (Rylie is like 5.5lbs). So I think that size would be perfect for your two. You'll just need to order an extra safety strap.
If you look at the picture of the Big pink one and the brown on on that link, you can see how the waist part of the seat belt goes around that back part (in between the spaces) just like it would go around a person's waist. This secures it to the seat really well - it doesn't move at all in my car. The safey straps clip around the waist-part of the seat belt and then you attach them to their harness.
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~Magnifique Yorkies~
Purchasing from backyard breeders, pet shops, and puppymills perpetuates the suffering of other dogs.
Educate yourself and buy from reputable breeders or rescue.