As @Umair mentioned, it’s possible to use an OmniOutliner document as an intermediary step between OmniPlan to OmniFocus. OmniPlan can export to OmniOutliner, and then OmniFocus can import that file.
Going the other way isn’t quite as simple, I’m afraid. OmniFocus won’t export to OmniOutliner, but if you have Omnifocus 2 Pro, then you can use Curt Clifton’s Export View to OmniOutliner AppleScript to create an OmniOutliner document that can then be imported back into OmniPlan.
It is important to keep in mind that while this workflow may be useful for some customers, it is best used for one-way data transfer, and doesn’t work well for round-tripping of data. If you are interested in seeing full collaboration between OmniPlan and OmniFocus, please email us, so we can add you to the open request!