Yes, you can. It's called Workspaces, and it's available to users with paid accounts.
You can have your own PushSend account for which you are an owner. Then other PushSend account owners can invite you to join their account as an Admin or Editor.
You can view all the Workspaces (additional accounts) you have access to by clicking the drop-down menu in the top right. Then you can switch between Workspaces by selecting the Company Name of the account you want to work on.