I understand that Omni are developing a Syncing function.

In the meanwhile, here is an applescript which copies whatever is selected in OmniPlan over to OmniFocus:

http://forums.omnigroup.com/showthread.php?t=15448

And here is another which copies whatever is selected in OmniFocus over to OmniPlan:

http://forums.omnigroup.com/showthread.php?t=15447

(There are various other ways of moving data back and forth without syncing).