I've got this working in my game. You'll have to prepare your audio a little differently than perhaps you're used to. For this to work, don't worry about clean cut-offs so that the two audio tracks join together perfectly. Leave in the overhang of the last note of the intro and have a good start to the main loop.
Use the system > compare two values expression. If Audio.PlaybackTime("intro") = 2.3, then play your main loop.
To do this, you'll have to have at least your "intro" audio file in the Sounds folder since the audio engine doesn't seem to be able to play multiple files from the Music folder simultaneously. You'll also probably need to preload on the previous layout to avoid a wait at the start of the layout.