This is how I do it to sync our iPhones...
This solution requires both MC and iTunes
(I don't have MC here at work so I can't check the exact option settings I have...)
In Windows create a folder for each iPhone to be your handheld
IN MC create a new Handheld device and point it to the newly created folder
In MC handheld options set the supported file type to mp3 (or something that the iPhone supports... I use mp3)
In MC Check the "Add Cover art to tags" when converting to get the cover art to iTunes
In MC set the number of conversion threads to reflect the number of cpu cores you have (We have a Phenom with 4 cores hence 4 conversion threads)
In MC I use the default playlist format (which I don't remember what it is...)
IN MC create a playlist to hold the files you want to sync
In MC Send the playlist to your newly created "virtual" handheld
(repeat for the other iPhone)
Open iTunes
In iTunes you should probably set "Manually manage the music etc" and "Copy the files to your iTunes library"
(Perhaps one could create a new iTunes library to be the handheld folder... My experience with iTunes is virtually none...)
Navigate to the windows folder where you created the handheld
Open the Playlist folder and drag the playlist-file (it has the same name you gave it when you created the playlist earlier) into iTunes and drop it somewhere over the Playlist-part (lower left corner in the iTunes windows)
iTunes copies the files to the music library... and you get a new playlist.
Then you can drag the playlist to your phone (you must connect the phone to itunes first...)
Your phone get synced. Hopefully.
I probably forgot to mention something, sorry for that
This method is not ideal nor the only one. Haven't had the time to experiment with other methods.
HTH John