This isn’t possible. Sonos devices aren’t designed to play music simultaneously with Echo devices.

They might be able to play simultaneously if you would use different Spotify accounts. Keeping them in sync will probably be impossible, though.

Why even probably; the sync needs to be perfect to extent of this being impossible to achieve by playing Spotify in this fashion.

The only solution of sorts for OP is to group the same makes by rooms, so that in sync play is as perfect as it needs to be in each room, and reliably and easily achieved every time.

Thanks all!  I was hoping it was possible since Alexa has a Sonos skill but I guess not.  I don’t have outdoor speakers and wanted to move a couple of Echo’s outdoors temporarily for an indoor/outdoor party and have seamless music between my indoor Sonor speakers.