I think combined is the wrong word. I mean, to place both domain's (sites) on the same server.

i just enabled memcached in 2.3 after having disabled it for a while and can see a 100ms + reduction in processing time for MYSQL.

Namespace can be any string, but I would avoid special characters.
You should feel improvements the longer memcached is running, as it will cache more entities. There is a stats table in admin overview somewhere

Few glitches but this great plugin is still working fine for me https://github.com/AU-Landing-Project/au_subgroups

Thanks a lot