To achieve this, you need to cheat - by actually using two or better three pages, loaded into separate frames.

|--------------------|
|Framed 1            |
|(containing Form1)  |
|--------------------|
|Framed 2            |
|(containing Form2)  |
|--------------------|
|Frame 3             |
|   submit button    |
|--------------------|

The submit button in page 3 will execute two submits as shown in the following code snippet. This button doesn't haven have to be part of a form. In fact, it could be an image or a regular button or even just a link with an href="javascript:..." tag.

<input type="button"
onClick="parent.frame1name.form1.submit();parent.frame2name.form2.submit()">