Can you use the Finesse desktop in parallel with a custom agent desktop for the same agent?
No, you cannot log in the same agent into the Finesse out-of-the-box desktop and a custom agent desktop at the same time. Currently, Finesse supports only one connection per user to the Finesse Notification Service. The Finesse out-of-the-box desktop counts as one connection, so when the same agent logs into both desktops simultaneously, that is considered two connections.
The agent can have the option of logging into either one of the desktops, but making sure that only one desktop is in use at a time. For example, agent1 can log into the Finesse out-of-the-box desktop and use it for a period of time. Then they can log out of the Finesse out-of-the-box desktop and log into the custom agent desktop. That would work fine.