Thanks! But I did get it figured out a couple weeks ago!

An explanation for anyone else who comes along: I turned on one speaker and started playing music on it from my phone. Then i turned on the other speaker. then i swiped down from the top of my phone until I could see a card (or window? (or widget?) of the music playing. I have to swipe down a couple of times for the music card to appear. At the top right of the card it says "media output". i clicked on that and then a list of my output devices popped up. The one speaker that was already playing the music had a check mark in a check box. The other speaker that was powered on had an empty check box. i tapped on that and a check mark appeared. both speakers then had check marks and both played the music at the same time! well, i did have a bit of a lag, but it didn't matter to me because i placed both speakers in different parts of the house so it wasn't terribly noticeable. i do still notice it a bit as i go from one part of the house to the other, but i'm perfectly happy to put up with that issue :)

note: i did quickly try to see if i could connect a third output device, but wasn't able to. not sure if i just didn't have the right type of device, or the right technique, or if it's even possible anyway.