It is just a two car garage. I do find myself moving between the driveway, side yard, and garage frequently. I mostly am working on cars. I have the occasional situation where I would like to load up a cart and strap it in the back of the truck to work on someone else's vehicle when they are in a pickle, but I do agree with floor space the top box seems more reasonable.
Here's what I do and it sounds like we're in a similar situation. I am in a two car garage but went this route.
HF 44" bottom (top to come)
26" Husky top and Craftsman bottom (full depth top)
Chest of drawers from Goodwill (all solid wood)
Craftsman 3 drawer travel box
Homer Bucket with Husky tool organizer
I am rehabbing a house and I work on our cars, so I pretty much have everything covered.
Bucket - I use the bucket for the outside around the house jobs, it's great as it's easy to carry, and you can even get a seat for a top.
3 drawer Craftsman - The travel tools go in here. I bought a separate set of screwdrivers, pliers, and socket plus some other stuff that you'll always need when fixing stuff somewhere else. I have a small tool bag for job specific tools. Saves time and the "dooh! I forgot to pack that scenario"
26" stack and 44" bottom - They live in their permanent location
Chest of drawers - I use this to store things like nail guns, knee pads, clamps, ect. It's full and has a permanent home.