Reading this post made me wonder - would be a real challenge to hike Whitney AND do R2R in the same day. (Would also mention that in the summer, Arizona is on the same time as California). Looks like the distance betweeen Whitney Portal and the Grand Canyon is 490 miles for the North Rim and 520 miles for the South Rim. Seems like this drive could be completed in ~7.5 hours (per Google maps). Super fast hiking times for Whitney Portal and R2R are both around ~8 hours.
So conceivably, a day could look like this:
12am-8am: Whitney Hike
8am-3:30pm: Whitney to GC North Rim
Would be a fun challenge - wonder if it's ever been attempted.
Like DUG wrote: chartering a plane would make the trip a lot more reasonable.