My mom has a shih'tzhu. Along with a jack russel, two german shepherds, and a small mixed breed she found on the side of the road. Shih'tzhus think they're bigger then they really are, and benefit from separate food bowls :P Also... getting them fixed since they're both 1 year might solve your problem entirely. At that age, male dogs tend to start going through puberty. And they start testing boundaries.
My parents are going to give their shih'tzhu to us this coming December. I have no idea why... I have a feeling their newest addition (the small mixed dog) and him don't get a long very well. It's strange because he gets along with EVERYTHING.