Not true at all. This has not been true for at least the last 2 series, and I have a good feeling it was never true, and only a myth that spread through the internet.
I have the EVGA Signature+ and an MSI reference 680. The Signature card has a factory OC, and the MSI one does not. I keep MSI Afterburner running in the background to keep tabs on their clocks, and they operate at their factory clocks. I have OC'ed the MSI card so they will operate closer to the same speed, but their boost tech makes it so they are usually at different clocks anyways.
I have tested this with 680's, 470's, 6950's and 5870's. None of those cards will sync their clocks when in CF or SLI. I've had other test this, and theirs also do not sync. It is just a myth that they sync up, or outdated information from back a few years.
The only issue you are going to run into is the fact that the cards fan profiles will be very different, and MSI afterburner does not allow you to tune the cards fan profiles independently. This may not be a big deal if you don't plan to change those fan profiles, but if you are one who likes to run the profiles aggressively to get the temps down, you will find the cards coolers will make it hard to do so, as they will need different profiles to be optimal.
Can I mix and match graphics cards from different manufacturers?
Using 180 or later graphics drivers, NVIDIA graphics cards from different manufacturers can be used together in an SLI configuration. For example, a GeForce XXXGT from manufacturer ABC can be matched with a GeForce XXXGT from manufacturer XYZ.
Can I mix and match graphics cards if one of them is overclocked by the manufacturer?
Yes. A GeForce XXXX GTX that is overclocked can be mixed with a standard clocked GeForce XXXX GTX.
Can I mix and match graphics cards with different sizes of memory?
When purchasing a second graphics card, you should try to match the memory size so that you are ensured full value and performance from your purchase. However, while it is not recommended, NVIDIA does offer the flexibility to run graphics cards with different sized memory by using CoolBits. Using CoolBits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate them together in SLI mode. When dissimilar memory sizes are enabled to work together using CoolBits, the effective memory size for each card becomes the smaller of the memory sizes.