I agree with Pat & Sharron - first thing is to make sure everything is going pretty well with the first one before adding the second. When I added my second, they became partners in crime - when one got into something, the other was only too happy to join in.
That said, I love having multiples. I actually found having 2 was easier than having 1. I didn't really find any extra work. I walked both together - pretty much they were a team. In my case, my kids breeder picked the perfect male (2nd basenji) match for my girl (1st basenji) and they knew each other well long before they came into my house - they came from the same breeder. The only time it was more work was when one was sick - then it took a little more management.
The added food cost was negligible, there is added cost for initial startup (dog, crate, bedding, toys) and vetting.
I now have 3 basenjis - I added a puppy (Aaliyah) last year. Having a 3rd hasn't always been the easiest and is definitely more work than 2. The times 3 have been a blessing was when Brando broke his leg this summer (Ruby had Liyah as a playmate) and when puppy Liyah was in heat this fall (I could run Ruby and Liyah together).
While I love having 3 - I honestly think having 2 is the perfect number (fun and really easy in general).