It's two way but predefined messages only. This way apple Apple has a map of messages to some minimal bit value and saves on bandwidth. Garmin InReach allows for predefined messaging too and custom as a pay per message option, it can take 10 minutes or more to send a custom InReach message. Also, InReach uses iridium satellite network which has true global coverage whilst Apple is using Globalstar, which doesn't.
The more I read about the new functionality the more it seems you not only send predefined messages to start the conversation with emergency services but you can send arbitrary text as well.