100 amp box with two 100 amp breakers
The sum of the breakers in a panel means almost nothing.  As long as all the wires attached to those breakers are suitable for 100 amps, you can add as many as you want.

However, if your service and main breaker are 100 amps, you are still limited to 100 amps total.  If you are adding enough large loads in the garage to need a 100 amp subpanel, you may regularly trip your main breaker and shutoff all your power - which means you would need to upgrade the service and main panel to provide enough power for everything.