You can use the Sonos app to group them together after they are already playing.
You can also create an Alexa Group. To do this:
Using the Alexa app, you can go into the Devices tab (last one on the right) and select the + at the top to make an Alexa group. Just assign it a name, an *Alexa-enabled device, and then the other smart home devices that are part of the group, including Sonos speakers. Once the group is made, tap on it from the main Devices page to set your Sonos speakers in it to be Preferred Speakers.
Please note, microphone enabled Sonos speakers can only be in one Alexa Group at one time for now, but the non-microphone enabled ones can be in multiple groups.
So in your case, you can make a group (call it something distinctive, like "Lounge and Kitchen"). Then use the process above to select the little speakers icons labelled Lounge and Kitchen. Then, at the bottom of the page, tap Preferred Speakers, and again select Lounge and Kitchen. Save.
Now, when you say "Alexa, play XXXX in Lounge and Kitchen" it will play in both. You can set up similar groups like "Upstairs", "Downstairs", etc.
*Note: It is not necessary to select an Alexa enabled device. This is only necessary if you want the device to always default to the group of speakers, without having to say "Alexa play XXXX in (room name)". Setting the Alexa enabled device eliminate the need for "room name".
See this link for a big discussion on Alexa Groups:
https://en.community.sonos.com/announcements-228985/sonos-now-playing-with-alexa-groups-6817588