In your movie m2 only exists on frames 1-3, but you're trying to access a variable attached to it on frame 5.
To fix this, you will need to either insure that both clips are on the stage at the same time, or you will need to copy the variable to a global or a _root variable, so you can get to it.
For example, if you modify the script of your m2 movie as follows:
_root.a = "Value I want";
Then you will be able to access it from the text field in symbol1 by referencing it as _root.a