Best to use the exact same cab as you have now. 8 ohm only: two cabs takes you down to 4 ohms. If you cant find one the same, try at least to get something very similar. Don't worry about splitting watts, it is going to be louder; more surface area, more air moving. Less work for each driver.
*1962 Jazz. '74ish Ampeg V4B, 115/210. * '75 Gibson G3. *Epi Tbird. *Squier: VM Jazz, CV 50's P. *Squier VM Jazz Assoc. *MBC 641. Squier owners club