Let me see if I have this correct:
Say you have two buttons and two fields. Button one adds the text: "Mary had a little lamb". Button two adds the text: "Her fleece was white as snow.".
If you have the cursor in field 1 and you click button 1, you see "Mary had a.." appear in that field. If you click button 2 you see "Her fleece.." appear in that field. If you put the cursor into field 2, the same two buttons add the same text, but to field 2 instead of field 1.
Is that what you have in mind?
If so, you can use Insert Text script steps to add the text specified in the script to the current field. (the field with the cursor in it.)