I believe both buildings must have a fully compliant grounding electrode system. Are these building existing or yet to be built? A concrete encased electrode at the garage would be the way to go in new construction. If the buildings are existing then there will need to be a minimum of two rods at each building if they don't have a concrete encased electrode. If the house has an underground metal water pipe then the two rods are sufficient as supplemental electrodes. If the garage has no electrodes other than the driven rods see if you can talk the contractor into a ground ring. Since the ground ring takes extra work and is not in fact code required you might suggest bonding the two sets of rods together via number two bare copper in the bottom of the feeder trench. I have often spaced the four rods that are required for two buildings equally along the feeder trench and connected them to both grounded busses in the two buildings using bare number two copper. Since twenty feet of number two copper encircling a small building such as a guard shack or pump house is code excepted as a stand alone electrode I figure that running the number two in the bottom of the feeder trench cannot hurt. One way I have saved a few dollars is to use bare number two run with the individual single conductor UF feeder cables in the trench between the two buildings as both the feeder equipment grounding conductor and the Grounding Electrode Conductor to the two sets of ground rods that I normally space out evenly along the bottom of the trench. The only place this would be a code violation is were the soil is corrosive to bare copper or there are livestock present on the ground above. If raceway is used for the feeder conductors then the bare wire in the trench can only serve as the grounding electrode conductor for the two buildings because the code requires that the Equipment Grounding Conductor be in the same raceway or multi conductor cable as the remaining conductors of the feeder. The deeper the feeder trench is the better. Remember that the code requires that the grounding electrodes be below permanent moisture level were practical. This is why I drive my rods through the bottom of the feeder trench thus adding the depth of the trench to the total depth of the driven rods. I have also driven rods through the bottom of the water line trench because in many portions of the country it's deeper than trenches for wire in order to protect the pipe from freezing.
One thing about the size of the feeder to the house. Since the feeder will be the "the main power feeder to a dwelling unit" you could reduce the feeder size to 2/0 copper under section 310.15 (6) assuming that the conductors are copper. If the 100 ampere panel at the garage is really warranted and the houses calculation would lead to a two hundred ampere service then the 3/0 service entry conductors may be undersized as they are only good for 225 amperes in a dwelling service.
The service conductors to the house could be run directly from the meter without a disconnect in the garage under section 230.40 exception No. 3. If they do that there should be no connection between the two buildings grounding electrode systems as it would be a parallel path for the neutral current under normal operating conditions.
230.40 Number of Service-Entrance Conductor Sets.
Each service drop or lateral shall supply only one set of service-entrance conductors.
Exception No. 1: A building with one or more than one occupancy shall be permitted to have one set of service-entrance conductors for each service of different characteristics, as defined in 230.2(D), run to each occupancy or group of occupancies.
Exception No. 2: Where two to six service disconnecting means in separate enclosures are grouped at one location and supply separate loads from one service drop or lateral, one set of service-entrance conductors shall be permitted to supply each or several such service equipment enclosures.
Exception No. 3: A dwelling unit and a separate structure shall be permitted to have one set of service-entrance conductors run to each from a single service drop or lateral.
"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison