In the callback for the push button:
str1 = cellstr(get(handles.listbox1, 'string'));
val1 = get(handles.listbox1, 'value');
if isempty(val1)
selection1 = '';
else
selection1 = str1{val1};
end
str2 = cellstr(get(handles.listbox2, 'string'));
val2 = get(handles.listbox2, 'value');
if isempty(val2)
selection2 = '';
else
selection2 = str2{val2};
end
combined_selection = {selection1; selection2};
set(handles.textbox, 'string', combined_selection);
