A creature can be under the effect of both spells at the same time. When CIP states "creatures can only suffer from one instance of Expose Weakness and from one instance of Sap Strength at a time", this means you could not have multiple sorcerers casting these spells on the same target to stacks buffs. debuffs. eg 2 sorcerers couldn't make a creature deal 20% less damage, if the spell is cast again it would reset the spell on the creature to deal 10% less damage and last another 16 seconds.