You probably want to use MC's Handheld Sync system:
http://wiki.jriver.com/index.php/SyncThis will allow you to specify what files or playlists get copied to each drive when you perform a Sync. I don't know of any way of trying to combine the two drives in your handheld into one logical drive.
But MC can Sync based on playlists or smartlists. One of the many smartlist options is a size constraint. So you could build a smarlist definition, based on some criteria, and at the end of it, say "limit to 200 GB" and MC will do that for you. Then, make that smartlist the sync target for your 200 GB card on the player.
I've gone down the path of trying to sync a large music collection into a player that wouldn't hold all of it. I had very elaborate plans and goals for this, in trying to fit in as much as possible, but have some randomness and some control.
What I found was that a simple approach was much better for *my* needs. ...and for me, this simply means a series of manually constructed playlists with bunches of my favorite albums in them. I just check off the few playlists I've built for mobile use and when I sync, it copies over the playlists and all of the files contained in them. Very simple and easy to maintain.
If you have some criteria in mind, we might be able to help you build some appropriate smartlists or something else to help you out.
Brian.