Can you please try the following steps and let me know if it works for you . You can try applying styles to the text container instead of selecting text ( span ) and styling .
Steps:
1 . Create a GWD Ad / HTML file
2. Select the Text tool and draw a text container on stage .
3. Type some text " Hello"
4. In PI add element ID "text1"
5. Select the Text tool and draw a text container on stage .
3. Type some text " World"
4. In PI add element ID "text2"
6. Select both the text container and convert it to a group
7 . Add a group Id in PI " TextGrp"
8 Right click on the group to Add Event
TextGrp>mouseover>Setstyles>Textgrp text1 > background : Green color Yellow
9 . Save and preview in browser
I have a attached the sample file for your reference
I hope this helps .