You can use only one cache implementation at time. This has couple of drawbacks:

• An instance without any implementation modules is broken (we should introduce a null cache factory to fix this)
• If you're using memcached impl:

1. You have to run two memcached servers.
2. You cannot use caching arbitrary objects without configuring too many memcached servers.
3. If you have ehcache and modules which use caching arbitrary objects (Silverpop, External traits...) and then you install memcached, caching arbitrary objects is broken.