Try calling SetSendMailShowing(true) before the SendMail(). That's one of the functions that get called when you click on tab 2.
Clicking on tab 1 calls SetSendMailShowing(false).
That function doesn't seem to help unfortunately. If I had to guess it's because even if that sets the tab to the second tab it doesn't refresh the mailframe so the second tab isn't actually loaded. When I try /script SetSendMailShowing(true) nothing happens.
If I might bump this with a related question, I seem to be having trouble calling SendMail from a script of mine. I can load that second tab using MailFrameTab2:Click(), and perform all of the other operations I want to (attach some items), but when I actually call SendMail, it accomplishes nothing. I can see from a secure hook that the call is being performed with all of the right arguments, but nothing actually occurs.