canon mount lenses will NOT fit on nikon cameras and vice versa. if you wnat to use canon on nikon you will need to buy adapters. but you lose autofocus capabilities and perhaps even metering in some instances
lens mount are exclusive to each brand of camera. the size is different and the point for electrical contact is also different. if you have a nikon, you need a NIKON MOUNT lens ( though not necessary nikon lens cos 3rd party like tamron, sigma etc also produce lens w mount for nikon, canon etc.
however, there are ways to 'cheat'. you can get adaptors that will ' change the mount of canon to fit a nikon lens for example. thought the mount does fit, the lens wil not be able to 'communicate' with the camera as the electrical contact will still be different.
tamron used to solve the problem during the Manual focua era by making lens and selling a suitable adaptor mount depending and what camera you have. but when it comes to the AF era, this method wont work anymore cos of the electrical contacts
When u buy a thrid party lens, u have to specify which mount you wan it for, canon, nikon, minolta etc
The mount is fixed onto the lens. You cannot change the mount without reconstructing the entire lens. Hence when u say, buying 2 mounts for your 3rd party lens, it effectively means buying 2 lens, one with a canon mount and the other with a nikon mount.
However, if u wan to buy 1 lens and be able to use it for both your canon and nikon camera, u will have to buy an adaptor as what many folks here have been advocating. Of course, u will lost AF.
Which lens to buy will largely depend on which camera u use more often. If u use the canon more often, than it would make more sense to purchase a lens with canon mount and find an adaptor to attach the same lens to your nikon body.