Yes. Whenever an ability is triggered, it's placed on the stack and will resolve as soon as it can. Before the game can move on to the next step or phase, the stack has to be empty. So, when you attack with Hanweir Garrison, its triggered ability will go on the stack, resolve, then the tokens entering will trigger Thalia's Lieutenant, once each. This all happens during the Declare Attackers step of the Combat phase.