I've just checked and you are correct - this is a bug. I'll make sure it doesn't cause any problems and fix it for the next release of Okra.
For the meantime you can make this fix yourself by downloading the latest release from the CodePlex downloads section. In the 'Okra.MEF' project locate the 'ViewModelExportAttribute' and set AllowMultiple to true on the AttributeUsage attribute (it is currently
set to false).
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
Hopefully this should then work,