How best to work on same song on iPhone + iPad?

[yep]

I am about to add an iPad to my iPhone DAW.

1) What's the easiest way to move songs with all the track info back & forth?

2) Do I have to buy DAW again?

Thanks for the help!

[pwnified]

You can download a Project Zip from each songs web page, and then upload it to the new devices (Songs On MultiTrack) page.
It's also possible to restore backups in iTunes onto other devices.

To get MultiTrack onto the new device, you can 'transfer purchases' to iTunes, and then sync them to the iPad. Or, you can repurchase it on the iPad, and you wont be charged again if you use the same iTunes account.

[yep]

Great - thx alot.