That is an IS schedule feature. Select the several columns over which you want to have a single header in this example "SIZE" then click on the 'Header Options' button at the middle top of the Schedule box and select "Insert header cell above".
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"