I am adding a 3 bd apartment and garage to my house. I would like to run 400amps to two meters that would go to 200 amp square d service panels. 1 panel will serve the new apartment, the second will serve the garage and I hope to run a line from this to the old service panel in the original part of the house. Thus, eliminating the old meter.
Can I do this, what equipment do I need to purchase?

Please consider contacting one or several licensed electricians to do the planning and possible work for you. The list of needed things is quite high.

The area of the apartment (using outside measurements and then subtracting for any space that can not be upgraded to living space.) to calculate the minimum number of 15 or 20 amp circuits for general lighting and receptacles.

A minimum of two 20 amp circuits for small appliance circuits in the kitchen. I suggest an independent 20 amp circuit for the refrigerator

The distance of the service entrance cables (distance from POCO drop to meters. Where will the POCO let you install the meters.

For example, you must have an electrical permit. Since you are planning on putting the service feeds to an apartment, the LAHJ may require that this be done by a licensed electricians.

Also, I would suspect that 200 amps per unit would be sufficient for just about any residential need, but you might want to do a load study to be sure.

Then you need to know exactly what the local Power company is willing to furnish you. Will you be using overhead drops or lateral service feeders?

Then you get to the specifics of all the internal wiring, particular inside the apartment. Are you familiar with the NEC codes for both apartments and garages.