If I use "Book and Upgrade" I get scuppered right from the get-go. There is no way to book anything other than a simple return. There's no place to putmultiple flights and when I click the box for one-way, thinking I could just do two one-way flights, the check boxes for upgrades grey-out.

Am I missing something? Doing it the wrong way?

Edit: If I ignore the fact that I shouldn't be able to proceed, it will do a one-way and show me the MFU at the end of the booking. Not really what I wanted to see money-wise though - I did a MFU round trip LAX-AMS and it was about $700 cheaper than just the one-way.