That's a lot of money for an 8 ohm resistor, a switch and a connector or two. If the original 2x10 is 4 ohms, that means it has two 8 ohm drivers wired parallel. Many heads will run into 2 ohms these days (my two will), so you could drive the additional cab, but if not, I'd look at an additional power amp to drive the other cab. Power amps are quite cheap and compact these days now we have class D. Alternatively, if you don't mind something larger/heavier, an old school power amp can be had for peanuts used. Gives you more flexibility.