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I'm planning to build a new detached garage. My question is can I add a second 100 amp breaker to my existing 200 amp Square D residential panel box? The existing 100 amp breaker goes to my quest house and the new 100 amp breaker would go to my new garage. Wire for garage would be ran in 1-1/2" sch 80 with a total run of 100 feet (panel to panel).

The 200 amp panel is feeding a single family home 3/2 about 2300 sf and the existing guest house of 800 sf.

Watch your stab limits

Sure, you can fit as many 100A breakers as you please. What you cannot do is place them opposite one another so they clip onto the same bus stabs.

Those little bars of metal, which are typically shared by breakers across from each other, have "stab limits" limiting the max current you can put on any one of them. The stab limit may be discussed in the panel's labeling. If not, make sure the breakers opposite them are smallish, like 15 or 20 amp normals (not tandems).

Note that it is common and normal for the individual breakers to total far more than the main breaker. That's because they're not all used to capacity at once.

Don't oversubscribe - but electricity is for using

200A service is more than enough for most homes. Because of that, you have some headroom for things like cottages and shops. And what's more, these days so many people have money that lots are putting in 100A subs on a whim, when in the past they might have fit a 30, 40 or 60A sub. So I'm not particularly worried about these multiple 100A subs. However, it is possible to overdo it, and excessively load your panel.

You haven't given us nearly enough information to know if you are doing that. That is determined by practical load calculations which would evaluate all your potential loads. But it is something to be mindful of.

For instance if I had a lovely wood shop in my garage, and wanted to add a big L3 EV charger, I would simply install the charger so the EV must be pulled into the wood shop area to charge, preventing its use.

Remember though; electricity is there to be used. If you need to, you can always increase your service size.

One last thing: think BIG on the panel

A guy sold us his entire shop for $5000. Suddenly we had eight 240V tools, and I needed 16 spaces in the panel. The moral of the story is: Spaces go fast! Get loads of 'em!

I was certainly glad I installed a 30 space sub panel, however suddenly I really wish I had installed a 40 space. I have backed away from advising 40 space subpanels simply because it seems so absurd, but it's not. Note that 30-40-space subpanels are not made in 100A sizes. You will be getting a sub panel rated for 150-225A and simply feeding it with a 100A feed breaker back in the main. That is fine.

Yes, the far breaker will trip first, but it's gonna do that anyway even if they are of equal trip value, since consumer breakers are too cheap to coordinate. Anyway if you really, really want coordinated breakers, use a shunt breaker as your sub panel main and a little Arduino or Raspberry Pi listening to a current transformer, and write code to have it shunt-trip the breaker before the main would trip.

"Can you?" Sure. But as Harper (above) mentioned, you run the risk of over-subscribing the panel. That means that the combined demand of all of the breakers, in normal operation, exceeds the capacity of the main breaker.

You write that the 200-amp main breaker currently feeds your 2,300 sqft home, an 800 sqft guest house, and now you're proposing adding a detached garage. That sounds like it could quite easily be over-subscribed.

Alternative solution: is the proposed detached garage near the guest house? Have you considered having the utility drop a second meter to serve the guest house and the proposed detached garage?