The best way to do it is always to run the short action cartridges in a short action chassis so you can use magazines designed for that caliber. If you wanted to save money up front and use them both in the same chassis then you will want to run a 300 WM magazine for the 7 Rem mag, and a 3006 magazine for the 6.5 CM. With the 3006 mag you will most likely have to tune the feed lips a little so it will feed correctly.
You could also try making a spacer kit to fill the back of the magazine. This would require some modifications but I have heard of customers doing it. It would be the same concept as a BR spacer kit in short action magazines (link below).
Shooting 6BR or one of the 6BR improved calibers has become extremely popular in practical/tactical matches over the last few years. They are easy to load
www.bisontactical.com
Remember with chassis you can always swamp components back and forth quickly. If you want to hunt with the 7 mag and train with the 6.5 Creedmoor then get the long action chassis set up exactly how you want it and buy an additional short action chassis block. Then you can swap the buttstock and grip over to your trainer chassis and you will only be out an additional $375 for aluminum or $575 for magnesium.