Are you a co-host, @Agustina-Y-Claudio0? I do not see a listing on your profile. If you would like a more in-depth respond you could provide a link to the listing.
If I understand your posting question, it is possible to make a separate listing for the couch. This way the guest could opt to use both the bedroom and the couch. Your couch listing would be the price you would charge for use of the couch. Alternatively, you could just state that if both the bed and the couch are used there is an extra fee of XX. You can request the extra fee on the reservation option "send or request money"
Hope this helps!