LiveLink™ for Excel® connects Excel with a COMSOL Multiphysics server, started in graphics mode. This is called a graphics server. The graphics server does not support multiple clients connected at the same time. To share a model between the COMSOL Desktop and Excel you need to start a regular COMSOL Multiphysics server when opening the model in Excel. See
Preference Settings to set the connection between Excel and a COMSOL Multiphysics server without graphics.