Is it the case that Snapcast server on RPi #1 will take the /tmp/snapfifo and pipe it to the 3.5mm output on RPi #1?
And for RPi #2, the Snapcast client discovers the server as it is on the same network (as I dont see any mention of config to point the client to the server) and the client also pipes the /tmp/snapfifo on RPi #1 to the 3.5mm on RPi #2?
Generally, only the Snapclient will play audio. This means you have to install a Snapclient also on RPI #1, so that it can connect on localhost to the Snapserver.
You could configure the IP address of the server, but the client will find the server automatically, because the server broadcasts the host name, port and ip address via Avahi ZeroConf.
The easiest way to setup a snapclient is the "Install debain package" option (https://github.com/badaix/snapcast#install-debian-packages).
Actually, the quickest way is to install the Snapclient on Android. An early prototype can be found here: https://github.com/badaix/snapcast/files/77482/Snapcast-0_4_9_alpha.apk.zip (see: https://github.com/badaix/snapcast/issues/9)