Cut them lengthwise with a chainsaw or band saw ASAP making sure that
you cut out the very center. Sealing the faces won't hurt but is not
as necessary. Cutting out the pith/center and sealing the ends is.
If you leave them whole they will crack from then ends for sure.

This link shows is a photo of Bill Lancaster ripping logs for bowl
blanks. You can see the blanks in the upper left of the photo:
http://www.heirloombowls.com/process_03.jpg

If you can swing it get a copy of Bill Grumbines first video on
turning a bowl. He is methodical on taking you through the process.
Here is the link:

http://www.wonderfulwood.com/

This process is also outlined at the a website called Around The
Woods. The link is below. Scroll down the page and their is a short
video to watch.

http://aroundthewoods.com/firewood01.shtml

If you leave the logs whole and let them dry then the likelihood that
they will split is much greater. Another possibility is to learn to
rough them out as a thick bowl first, seal them and then let them
dry. Or you can go ahead and turn the bowls thin right away.

Good luck,
Ted