Hi Lisa,
I presume that you are using LiveCycle Designer to develop the forms and then extend/use the forms in Acrobat.
I have put two buttons into a table cell (one directly below the other), but you are right it is not possible to overlap them.
When scripting you should use double quotation marks.
I am trying to understand your form. When you say that the button caption is re-setting to the default, do you mean that the button says "Button"? You can change the default in the Object/field tab to "Yes", or whatever else you want. You decide what the default appearance of the button (font, colour, etc.) will be, not Acrobat.
I don't think that you are doing anything wrong in Acrobat. The issue is in LiveCycle. Select the button and go to the Object Tab, in the caption box change "Button" to "Yes". Then in the Accessibility Tab add your tool tip. This should solve the problem (I hope).
Of the two possible solution I would recommend the single button, invisible numeric find (in an adjacent narrow cell) and the scripting solution, because you are putting the button into a table.
I have put a sample on share,acrobat.com:
https://share.acrobat.com/adc/document.do?docid=5cfe3a71-4ad4-44bc-838d-9ec8a2014dbf Hope that helps,
Niall