two devices and one account

[iluv2golf]

How do I use Spotify on two devices.  I do not want to use them at the same time I just want to be able to use my account from whichever device is handy.  I do have a premium account and it works fine on my iPhone.  I would also like to use it from a second iPhone but, when I try to log in from that device it states that the account has already been used.  I know, it is my account!

any help is appreciated