Separate names with a comma.
Even the personalised ones? Gnmarechal has one that says that his messages are sent from "his cave of despair where he collects souls"...
Yes, you can customise it. The sent from is the default one.
Mine originally said "Sent from my Nexus 4 using Tapatalk", then I switched to "Sent from my Nokia 3310 using Tapatalk", then the DynaTAC 8000x, and now this.