Counterspell
If the players are within 60 feet of the BBEG when they enact this plan, the BBEG will want to cast counterspell. Since the player characters are readying an action to cast a spell, it should be obvious to any BBEG worth their salt that they are about to enact some nasty combo and that they should stop that.
However, the BBEG only gets one reaction, so they can only counter one spell. There is the choice of countering wall of force or sickening radiance. They can only stop one spell. If the party did not coordinate their actions, they may have allowed the BBEG a turn between the two casters such that the BBEG could counter both spells. Thus counterspell rewards the players for good teamwork while preventing them from trivialising the encounter.
Note that if Readying a spell is involved, counterspell must be used when the spell is Readied, because that is when casting takes place.
Dispel Magic
If the BBEG is hit by this combo, they can cast dispel magic on sickening radiance. They are still stuck inside a wall of force, which is not a good place to be, and they may have taken damage from one round of sickening radiance if they failed their Constitution save. The BBEG's escape will be sufficiently delayed that the party might gather into a more advantageous position.
Disintegrate
If the BBEG is high enough level to have disintegrate, then they can use it to get rid of the wall of force. They can then walk out of the sickening radiance, although they'll still have faced at least one round of sickening radiance (depending on their speed). Forcing the BBEG to use their disintegrate on a wall of force and not a player character is a substantial advantage.
Harm the casters, break their concentration
The BBEG might have some way to harm the characters without attacking them directly. BBEGs regularly have minions (and if your BBEG doesn't have minions, why not?). The BBEG would order their minions to focus fire on the two casters, attempting to break their concentration.
However, you specify solo BBEG. If adding minions is not an option, you can instead consider adding lair actions. Lair actions will function as long as the BBEG is alive and can be used to affect the party even if the BBEG can't reach them personally.
Until such time that concentration is broken, though, the BBEG is trapped, unable to fight directly, and making saves against sickening radiance. Depending on circumstances, it might be many rounds before the BBEG can escape. This rewards the party's ingenuity.