Is this fair and allowed in tournaments by FIDE?
The first thing to note is that FIDE only directly sets these kind of rules for tournaments which are directly controlled by FIDE like the Olympiads and the World team championships. For other tournaments these rules are set by the tournament organizers.
So, if your local or national league team competition allows this then, even if the event is FIDE rated, FIDE doesn't care. The same is true for continental team championships. So for the European Club Cup you would need to check the rules set by that governing body.
For FIDE controlled team events the rule is that the squad of usually 5 or 6 players must be declared before the event along with a board order which may not be changed.
According to the Regulations for the FIDE World Team Championship:
4. 1. Composition of Teams
4. 1. 1. Teams shall be made up of four main players and up to two reserves.
4. 1. 2. Each team shall have a captain, who may also be one of the main players or reserves. Each team’s definite fixed board order shall be confirmed by the team’s captain at the Technical Meeting (see Article 4.3). There is no requirement to assign board numbers in rating order. Each player shall be assigned a number from 1 to 6. This creates a board order, which cannot be changed and remains fixed throughout the tournament.
4. 1. 3. Each team’s composition shall be announced by the team’s captain not later than thirty minutes prior to the start of each regular match
So, you could have player A play board 1 and player B play board 2 when they get white on board 1 but player B could only play board 1 when they get black if player A doesn't play in that match.