At this time each Roku account can only be associated with one Pandora account, so you won't be able to use multiple Pandora accounts on a single Roku device. If you want to listen to two different Pandora accounts, you'll need:
two Roku devices
two Roku accounts (one on each).
If you have more than one Roku device and have already signed them into the same Roku account, you can manage your devices using the following steps: