To link channels, first go to the mixer window and click on one the first channel you want to include (being careful not to click on any of the controls or editable settings). Then Shift-click on the last channel to select all those between it and the first, and/or Ctrl-click to add other channels that don’t follow in sequence in the mixer. The channels you want to link should now be highlighted in the mixer. Now right-click on the background on one of the channels (somewhere near one of the faders is good — but avoid right-clicking the pan pots as this accesses a different function on stereo tracks) and select ‘Link Channels’. Now, when you select a different channel, and then reselect one of the linked ones, you should notice that all of the linked channels will be highlighted in the mixer. Even better, if you move the fader of one, you’ll see all the others move ‘in sympathy’ – you’re able, at the tweak of a single fader, to alter the level of your drums relative to the other parts, without interfering with the dry/effected balance.

 

Of course, you might not want them to be linked all the time, and if you then notice that you need (for example) to raise the level of the snare relative to the other drums at a later stage of the mix, Alt-clicking (Option-clicking on a Mac) on the snare fader will allow you to adjust its level independently of the linked channels, while preserving the link. If you’d rather unlink the channels so you’re free to do lots more tweaking, just right-click, select ‘Unlink Channels’ and you’re done.

 

Source/more info: http://www.soundonsound.com/sos/nov08/articles/cubasetech_1108.htm