Both the AF and the BA lounge will stamp your boarding pass in an attempt to prevent 'lounge hopping'
Well, a passenger can print out his boarding pass twice.
Moreover, a boarding pass on a mobile phone cannot be stamped either.
TopBonus Golds (on AB operated flights) apparently can also access the BA and AF Lounge in TXL. They stamp. However, nothing prevents the pax to print out the pass twice.