Thank you for your questionWhat you need to do is go to your local electrical supply house and ask for a 400amp meter main. This will have the meter on one side with spaces for breakers on the other. This will allow you to feed your home, and the garage.What you will have to do with this setup is change the wiring from the panel to the inside of your home, and change the wiring going to your garage. The wiring will need to be 4 conductor.Now question for you - how far from the home to the garage?
Ok the wiring to your garage is going to be a problem. It needs to be 4 wire. You will need to pull an additional grounding wire #4 copper and then install a grounding bar kit in the panel. Then you need to be sure you isolate the neutral and grounds in that panel and bond the grounding bar to 2 separate grounding rods spaced at least 6' apart.Here is how you do this:Now at the new service you need to get the 400amp meter main like I stated before and run 4" conduit up the mast to the weather head using (2) 500mcm cables and (1) 250mcm for the neutral.Again you will need to ground all of this like I show here:Just keep in mind that if you don't have a metallic water meter, you will need to drive 2 grounding rods spaced 6' apart.Now onto the house panel, the new cable from the meter main to the house panel will need to be 4/0 aluminum SER cable. And it will need to be installed like this:Just be sure to get the proper electrical permits and inspections co-ordinated when you do this. In this way you can get this inspected and re-energized the same day.