No, it’s unfortunately not possible at the present time. You’ve made assumptions that just are not what actually happens.
You have assumed that when you go to authorize SmartThings to an Amazon account, you will be able to select the location that you want to use. But it just doesn’t work that way.
SmartThings will present one and only one of your locations, and it will always be the same location every time. You can’t get to the other locations to authorize their use to echo.
The same problem comes up when trying to use IFTTT with multiple SmartThings locations on one smartthings account.
This has been discussed previously in the forums, it’s a known issue, everyone agrees that it would be nice if you could get to the other locations, but at the present time you can’t.
SmartThings allows you to have multiple locations on your account, but it does not allow external systems access to the additional locations.
So in order to give echo access to the devices belonging to your second hub, you have to set that hub up as a separate SmartThings account.
With SmartThings, it is best not to do any thought experiments based on what the logical design appears to be. you have to test it in real life to find out what will actually happen.