You are replying to a post which is over three years old and much has changed in that time, Specifically with the echo integration. For a while, it did work exactly the way the OP for this thread wanted it to work. However, they changed it again in April 2020 and now you can have multiple locations, but all of your SmartThings locations From The same smartthings account will always be exposed to a single Alexa account.
I don’t know if that meets your requirements or not: it does not meet the requirements of many customers. So they are working on changing it yet again.
Here’s how it currently works:
https://support.smartthings.com/hc/en-us/articles/207808076-Amazon-Alexa-SmartThings-FAQ#Locations
And here’s the discussion of what may change soon:
In the future, I recommend looking for threads which have had activity in the last 12 months. Otherwise it just keeps resurfacing really old stuff which then confuses everybody, particularly since smartthings features change so often.