Call Other MovieClip from within another MovieClip

yes.

are you using as3 or canvas?

what's the parent/child relationship between the two movieclips?  ie, they (or an ancestor) are both on the main timeline, correct?

if yes, they can communicate when both exist.  (eg, you can use code in mc1 that exists on the main timeline frame 4 to communicate with mc2 that won't exist until the main timeline frame 100.)

if no, explain.