Lots of comments here:
Would this run through the attic be in conduit? That makes a huge difference. If it is just NM (Romex) or SE type cable it must be limited to the 60c insulation rating limits and so that makes the sizing requirements much larger.
For 60c limits you would need 1 AWG copper to get 100 amps (it is actually rated for 110). 2 AWG is only good for 95 amps so it is not acceptable for a HPWC at 80 amps since that 80 amps is continuous which means you need 25% more ampacity. You could also do it with 1/0 aluminum which is rated to exactly 100a at 60c. You can't run aluminum direct into a HPWC, but you could take it to a subpanel or I think you can also just pigtail splice it to copper to get the last distance to the HPWC. You need 3 awg copper in conduit to feed the HPWC at the full 100a. I don't think it will accept any wire larger than 3awg.
So for your run to the garage from the main panel, if all in conduit you could do 3awg copper or 1awg aluminum. If NM or SE cable you would need 1 AWG copper or 1/0 aluminum.
http://www.barr-thorp.com/wp-content/uploads/2011/04/digest-176-nec-tables.pdf
Assuming you have enough available capacity to your house in general (you need to run load calculations) then a really good solution might be to run an aluminum feeder from the main panel to a sub panel (or even just a splice point / split point) in the garage, and then to run 3 awg copper in conduit to both HPWC's and have them linked by RS-485 to share capacity.
One benefit of having a panel in the garage is that it could count as the disconnect. For breakers feeding a HPWC at *over* 60a you have to have a disconnect readily accessible that can be locked in the off position. I interpret this as a "lock off" device in the main breaker panel is sufficient, however, some AHJ's (authorities having jurisdiction) want the disconnect close and within visible range, etc... You could also reconfigure later if you get a different vehicle charger that did not speak the HPWC coordination protocol. You could downgrade the remaining HPWC to a lower amperage (through settings) and have a second breaker in the panel for some other kind of charger or plug.
P.S. You also might need to do the calcs on correction factors for the cable running through the attic since it may get hot. Though typically NM and SE cable is actually thermally rated to a higher temp even though code says you can only use it at the 60c rating. The higher rating still applies for doing the temperature correction factors.