You will BOTH have to fill in a PLF, completed within the 48 hours before you arrive in Belgium. RonnyW gives the link above but it is convenient to do it on your smartphones if you have them, and the task isn't onerous. If you are flying you may well have to show that you have completed the PLF before you are allowed to fly.
If we are going to multiple cities do we have to fill out multiple plfs?
No just the one with your first address. They can contact you by phone if necessary.
"If we are going to multiple cities do we have to fill out multiple plfs?"
I would imagine it is only for the entire country.