Wire the speakers in parallel and you can do multiple speakers with this setup (I've done 8 at a time).

If you wire it up in series, you have to double the time on the variac (or triple or quadruple for 3 or 4 speakers)

Variac break in times for Scumbacks, Celestions are 6x longer: 

Speaker Imp	 	Speaker Power Rating	Break in Voltage		How Long?
8					65						13.165			 Light = 2 to 3 hours  
8					30						8.9				Medium = 8-10 hours
16					65						18.61			Full = 18-20 hours
16					30						12.64
8 					100 					 16.325 

16					100					23.094