Yes you can see more than one at a time!
Very Simple Method – download another browser – Chrome is good – and open the other account in that.
Classy Method – you need to use Google Chrome as your browser for this solution. Click on the spanner symbol (top right) and go to Options > Personal Stuff. Sign in as another user then switch between users. I’ve not tried this so let me know.
By the way export your contacts from one account and then import them into the other and you can keep them in sync that way. If you have a premium account they are kept in sync anyway.