you can channel 1 and 2 of a clip as a single item only if they are a stereo pair … select the tracks then use Modify > Stereo Pair command (Option-L) to pair them … now adjustments will affect both tracks of the clip simultaneously
if you don’t want to pair them, or they are multiple independent clips then you can make global adjustments to multiple clips or tracks by using the Modify > Levels command. First select all the clips/tracks you want to adjust then select Modify > Levels (or press Cmd-Option-L) to bring up the Gain Adjust dialog where you can make an absoulte or relative adjustment to all the selected clips.
good luck
Andy