You don't have to do this if you're hoping they'll bond with a different soldier but if you form the bond it becomes permanent, while they both live.

Once the joint cohesion bar for two soldiers has reached maximum for the first time, a level 1 bond will be formed and you'll be asked to confirm that you would like to bond the two soldiers. A solider can only have one bondmate at a time but if their bondmate dies they will, eventually, be able to form a bond with another soldier. Sending soldiers on missions with teammates with whom they have high cohesion with will increase the likelihood that these soldiers will become bondmates. Bonds work on a rating system named Cohesion, which ranges from 1.

If you need more help, our XCOM 2 War of the Chosen guide provides a breakdown on how else you can tackle the huge expansion. Each soldier has a compatibility rating with every other soldier in the squad, this rating is called Cohesion and effects how quickly one soldier will bond with another. XCOM 2 Bonds is a new system introduced in expansion War of the Chosen, which rewards you for having soldiers complete missions together.