The meter is in between my house and the garage. Can I connect the service conductors from both panels to the same lugs at the meter? Do I handle both panels grounding and neutral bars as 'mains'? There is no conductor running from the garage to the house.
In my State you cannot "double lug" inside the Meter. This is a requirement from the Utility Companies. Also you cannot connect two conductors to a lug intended for a single conductor, see NEC 408.41.
You can put in a junction box or wireway and tap the conductors in it. There are many insulated lugs that can be used. Just make sure you follow all the tap rules.
My preferred method is to install two enclosed breakers or a panel with two breakers. Which is allowed under the six disconnect rule and you can run a large conductor (200A) to the house and a smaller conductor to the garage (40A?) and not worry about tap rules etc. In your case, see NEC 230.40 Exception 3.