You can make this work, but be careful of your channel spacing.
If you use 40MHz-wide channels to get your best 802.11n rates, then because of the way 2.4GHz channels' center frequencies are spaced just 5MHz apart even though the channels are 20+ MHz wide, you'll end up taking up five full 2.4GHz channels and partially overlapping several others. If you choose to put your 40MHz N network in the middle of the band, you could end up at least partially overlapping every single channel in the band.
It would be best if you put your N network at one end of the band, and put your G network at the other end of the band.
Beware, though, that 40MHz-wide channels in 2.4GHz don't leave much room for other uses of the band. If you put a machine on a 40MHz-wide channel and try to use a Bluetooth mouse or keyboard or other device with it, the low-power Bluetooth radios may have a hard time finding enough frequencies to hop to that don't get clobbered by the 40MHz-wide transmission from the nearby Wi-Fi radio in the same machine.
Overall, I'm a big fan of limiting my N equipment to using 20MHz channels in 2.4GHz, and using 5GHz N equipment to do 40MHz wide channels for the best rates. Even at just 20MHz wide channels, 802.11n can 3 to 4 times the signaling rates of 802.11g.