Agree with cosmicma in that you probably could.
Question I would ask is Why? If you have a spare 555, or can pick up one real cheap, then I could understand but if you have to pay decent money for another 555, I can't see that as a cost effective upgrade. Wouldn't that money be better put towards upgrading speaker or amp?
Do your speakers have bi-wiring/bi-amping connections or will you need to separate the crossovers to create them?