Thoos.com - One of my favorite Rim to Rim to Rim Write-ups:

"Also commonly know to as the R2R2R. This run is not frequently done, but usually you can find a few people attempting it any April or October weekend. You can go earlier in the year (than April), but you may have problems with limited daylight hours and possible snow on the North Rim. Go much later in the year and you may have to deal with extreme temps in the canyon......." Enjoy: http://thoos.com/guides/grand-canyon-rimtorimtorim