in your field a script add:
set variable [ $$trigger ; 1 ]
in your field b script start with:
if [ $$trigger = 1 ]
set variable [ $$trigger ; "" ]
In your script step that modifies field B, use the Set Field script step. The Set Field script step interacts with the underlying data object, not the layout object that the script trigger is applied to, so the OnObjectModify trigger isn't triggered by manipulating the field this way.
Thank you! This, when combined with the "Hide Objects" feature makes for a simple way to stack fields and save space.