using em Client on two computers

Yes, one license per device.

The email accounts they have setup in eM Client are irrelevant. The license is tied to the device, not the email address they use in eM Client.

We have not yet concluded the license model for the phone app. It is still in beta, so there is much that could change.

OK, that is clear from a “licenses required” perspective. We will see how the laptops respond and if we need to set up forwarder emails for those extra devices in order to register them against users.