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Leaving Belgium on Orange card procedure
it very much depends on the nationality of your partner. If Belgian or some specific european nationalities (not all) you won't need any working permits. There is an office in the Gare du Nord building that can give you precise information about this.
http://www.bruxelles.be/artdet.cfm/5308
we had exactly your similar situation and no working permit was ever required (however potential employers request the cohabitation document)