It's not bullying all the time, sometimes laro sa kanila yan or sometimes "I'm the Alpha Shih Tzu here so don't mess around warning".

What you can do is watch both of them play. As soon as the male starts humping or bullying, put him aside right away, look at his face, and in a big, commanding voice tell him "NO". That way you're assuring the "pack" that you're the Alpha of them all.

You have to distinguish what's play and bullying. If they play well, praise the male with "GOOD DOG" and tap him.

Try putting them outside his territory (anywhere but the house. e.g. PARK, a friend's house) helps them to be at peace to each other.