There are four inputs on the powerbase. Two are for throttles. The other two are for mains adaptors. Any standard set is supplied with just one adaptor which will work fine for two cars.
Alternatively, you can use two adaptors. Slide the blue switch to the right and this will distribute power to both tracks independently. This means that you get more power if you have extremely long circuits (I have gone round large rooms with just one adaptor) and it also means that if one car de-slots (comes off the track) then the other car will not suddenly get a power boost.