shih tzu acting out..

Feed high value treats seperately. 

Put high value toys away unless you are there to supervise and play WITH them. 

If she jumps up when you are holding him, I wuold just get up and walk away - leave the room CARRYING Teddie without looking at or speaking to her.  Go back in after a minute or two and sit back down as if nothing happened. 

The mounting is likely not sexual; probably more to do with play.

It's not that she is being "horrible" to Teddie, she is just being a brat and it seems she is used to getting her own way. 

Put her on strict NILIF, like, YESTERDAY.  Stop giving her all the love, attention and priveleges she wants, whenever she wants them.  Make her EARN all the Good Stuff. 

She wants petting?  She has to sit first.  She wants to sit in your lap?  She has to wait to be invited.  She wants dinner or a treat?  She has to do a down/stay.  She wants you to open a door?  She has to sit.  She wants you to throw a toy?  She has to come or heel.  That kind of thing.  Use cues that you know she knows WELL - it may just have to be "sit" to start with.  This is a time to reinforce behaviours she knows, not teach new or rusty ones, that only sets you both up to fail.  Ask once only: "Coco - " (she looks at you) "Sit."  Pause, wait for her o comply... if she doesn't, she doesn't get that thing she wanted. 

If you are concerned about Teddie's reaction, DON'T risk it.  Intervene if she behaves in a way that you think Teddie will not like, simply by stepping between them and/or gently removing her, maybe crating her to allow her to calm down.  Don't do this if you are not sure of her temperament; the last thing you want is to give her a ite history.  Use a trailing line if you are concerned about her reation.  This way you will not have to worry about Teddie flipping out and attacking her, and you are also reassuring Teddie that he doesn't HAVE to, becuase you will always step in.  You are also reinforcing to both of them "hey - I'm in charge.  Knock it off."

Do they have crates?  Both would benefit from some down time AWAY from each other - and you - every day.  This gives Teddie a break from her, and will make it easier for him to tolerate her when she is being a bit bratty.  It's an ideal time to dish out high value treats or chews as well.

Mounting him is not sexual -- it's her trying to convince him SHE is the boss.

Very often a female will tell a male what to do -- likely she's gotten away with the "puppy" card for a long time, particularly since she wasn't spayed and he WAS neutered.  Now the playing field is more level -- she's ramped up being a brat and he may or may not tolerate it.

I supervise CAREFULLY (none of my dogs are ever "alone" together -- they are crated when I'm gone) but I do allow an older dog to give puppies a "knock it off" cue.  Nothing mean -- but a look, moving away, or even a grunt or low "behave yourself pup!" growl because they need to be able to put the pup in its place.

But I DON'T allow any real aggression at all -- like Chuffy says I move them away, step between, or verbally "No growling" -- whatever is called for.

I don't like humping -- we do pet therapy and it's just not acceptable behavior out in the world, so I will simply tell them to "knock it off" or separate them and distract.

When she gets riled up, direct her somewhere else and give both separate toys or something to chew on. 

Put a leash on Coco to prevent him to reach Teddy on these situations. He does it because he simply can get away with it. Teddy does not fight back (yet) and by running away he avoids you stepping up too. Just get the leash and if he goes after Teddy just use the leash to prevent it

I actually think the mounting is neither sexual or social (being the boss), but in reality this mounting sounds like classic anxiety and frustration. Mounting and humping is a strong indicator of an anxious dog who is often conflicted about another dog and him/herself, and doesn't really know what the appropriate response is in a given situation.

She just got spayed 2 weeks ago - her hormones are probably changing (along with her brain, as she's just heading into maturity now!), as it does take time for the hormonal changes of a spay to really start to kick in. She's probably feeling pretty out of sorts because spaying does change a female in a lot of ways. She's probably a bit stressed out, and these behaviours she's doing is not really that surprising to me. It sounds like she's pretty insecure and taking it out in bullyish behaviours.

If it just started in the last few days, I would guess (as long as you intervene now) that it won't last forever, that once she settles in to spayed life things should return to normal. However, in the meantime Chuffy has some good advice there.

The biggest thing is to look out for your male and ensure she doesn't get to keep bullying him. Prevent the mounting behaviour - there's no need for harsh punishment, just don't allow her to do it - keep interrupting every time she does it. I wouldn't even use a loud noise, just walk up and stop the interaction. A loud noise may be taken as unintended punishment by your male.

NILIF would be a good place to start with her, but to be honest it would be most fair for both of them. She needs to learn that she does not need to steal from him, to mug him, and to bully him. Especially if it stems from insecurity or vulnerability (which it sounds like it does, although without seeing it is hard to say if it's that or her just walking all over him...but with it just happening the last few days, after a spay, it sounds more like the former), she just needs to know that good things come from good manners, and that she can earn all those things she wants by doing other things.

I would also make sure they are getting sufficient exercise to ensure that she (and he) is getting what she needs so that while in the house she'll be more relaxed and satisfied, and perhaps less likely to be a bother. Little dogs may get a lot of their exercise indoors, but they really do benefit behaviourally from outdoor exercise daily.