I belong to two meshIQ subscriptions. Do I need two different accounts?

No, you do not need to sign up for two different accounts if you belong to two separate meshIQ Cloud subscriptions.  

The same meshIQ Cloud account can be used to log in to the cloud dashboard and to log in to multiple subscriptions.  These could include your own subscription and those to which an owner has invited you.

You use the same credentials to log in to all of them. However, your roles may be different on each. You may have the role of Owner for your own subscription. But when you're invited to belong to a different subscription, the owner of that subscription assigns you a role.