Cascade prompts with static choices

[handpass]

I have a finished report with one prompt page. This prompt page has two drop down list prompts.
In my first prompt I have 2 static choices. Depending on what is chosen in this first prompt I then want to display three more static choices in the second prompt, the values of which will depend on the first prompt selction. e.g.

IF Prompt1 = a
THEN
Prompt2 = a1, a2, a3
IF Prompt1 =b
THEN
Prompt2 = b1, b2, b3

I am hoping to have these two prompts on the one prompt page if that is possible. The values selected from the two prompts will be used as parameter filters in the report.

Thanks.