@Alin Andrei postou a resposta certa para essa pergunta. Estou citando ele aqui, pois a resposta está escondida nos comentários. O crédito vai para ele:
1. memcache and memcached are two separate extensions that do not depend on each other so yes you can have them both installed.
2. if both extensions use the same memcached server, then yes the memory is shared between them, you can keys from both extensions but only the size of 512
3. if you can use the same extension on both apps then use only one to simplify the complexity you have. You could both of them as well, i dont think it will increase in size if you use both of them