There is unfortunately no way to have multiple people monitoring the same user at the same time. In case you haven't tried it, you'll get an error when you try to begin monitoring with the second device. You could turn on automatic call recording, which would allow the second person to review the call later. To do so you'll need to have a Premium or Enterprise (Ultimate) account with us.
I apologize for any inconvenience this may cause.
I would suggest speaking with your Account Manager for a solution that would best suit your particular needs. We may be able to provide you a work around depending upon a few things. In the meantime, I'm going to flip this to a feature request for other members to vote on.
I think that RingCentral should allow multiple users to monitor the same call. At my company, when somebody is discussing a new job with a client, we want everybody who is going to be involved with that project to listen in on the call. RC's inability to support multiple users is a major hindrance to us getting everybody on the same page, and I think it would be best if you explore solutions for this.