There are a lot of limitations at the moment when you use different accounts on different Echo devices in the household. Unfortunately when using music services tailoring to different music tastes is very problematic. It would be great is Alexa allowed each device to use it's own music account but that is just not possible right now.
This is going to sound simplistic, but if the goal is to be able to allow each person play their Spotify on their Dot, then let them connect their (phone, tablet, Ipod) to the Dot via Bluetooth. They can use Spotify's app and the Dot just acts as the speaker. Everything else will act as normal on the Dot, but no, you won't be able to issue voice commands for the music itself.