Multiple-select has been specifically disabled when having internal subdomains/boundaries active (enabled with only external boundaries), as a user could then possibly select both internal and external boundaries which typically have different number and types of boundary conditions causing a mismatch (the Matlab GUI toolkit does not allow for multiple select by groups).
However, the "Continuity" condition should be the default boundary condition for internal boundaries and should not need to be selected manually.
You can of course also manually hack the listbox to allow multiselect with something like (
https://www.mathworks.com/help/matlab/ref/matlab.ui.control.uicontrol-properties.html#propname_Max )
set(findall(0,'Tag','list_seldom'),'Min',0,'Max',2)
but there is not guarantee that it won't lead to errors in Gui and model, in particular if you happen to select incompatible conditions.