When you stack 2 switches, the switch behaves a single virtual switch.
Every vendor has a proprietary way mechanisn of how the stack operates. They usually provide you with the stacking cable that need to be used for connecting them as stack.
Stacking between different vendor switches are not supported. The switches however can just be uplinked using any ports on the switch.
Stacking is platform dependent and is not inter-vender capable.
The 3750/3760 will stack using stackwise Plus technology, 3760 being the better due to the fact that the backplane is doubled to 64G and local traffic does not traverse the backplane. You need to connect the switches together using the stacking cables to daisy chain them together. One of the switches acts as a master and all management is performed from that switch ( even IOS as the slaves will upgrade there IOS from the master ). This can be troublesome when trobleshooting ASIC issues on non master switches but you can issue the remote command to execute commands on other switches in the stack.
The 6550 has a "stacking" ability as well, ( notice stacking is in brackets) which is called VSS but needs a VS Sup720 and a dedicated TenG port cross connect. Also I beleive that only 2 6500's can be linked at present but more are in the pipe line.
Junpier do a stacking system the can span Data centers which is cool as it works over ethernet.
As with previous post, stacking two switches together using the StackWise cable will not work due to the following reasons:
1. Mismatch/Proprietary software codes;
2. Pins don't match; and
3. What financial benefits would this bring if this was enabled?