I think you misread it.
After 20th level in a type of magic (druid, cleric, wizard, sorcerer, etc), levels don't give you any more spells per day.
I believe your caster level still goes up, and so does everything based on it such as the difficulty of dispelling your spells, range, damage (if it is below the damage cap), etc.
In the case of a Fi10/Wiz10, if they took their 21st level as Wiz they would now have a caster level of 11 and spells per day of an 11th level caster. Assuming they eventually made it that far, at Fi25/Wiz25 they would have the spells per day of a Wiz20 (ignoring any feats that may modify this) and a caster level of 25.