You must maintain the passive crossovers.
In a standard series wiring, the amplifier + goes to the first speaker's +, the first speaker - goes to the second speaker +, and the second speaker - goes to the amplifier -.
Now you may be getting confused by the crossover module, but nothing changes. Connect the passive crossover to the speaker terminals as normal. Treat the crossover +/- inputs as the only terminals when you do your series wiring. In other words, simply replace the crossover inputs with what would normally be the speaker terminals.
So the amplifier + goes to the first speaker crossover input +, the first speaker crossover input - goes to the second speaker crossover input +, and the second speaker crossover input - goes to the amplifier -.