It depends on where it's being tested and even who's doing the visual. There are some locations that are looking for a even replacement cats that mimick the factory set up -- that is, if they're looking for 2 cats on each side and they find only one, you've got a problem.
There are other locations that only care that the cats are there and that you pass the sniffer.
Beware - there are different cats for different jobs. If you're replacing 2 cats with one be sure it's a "3-way" cat you're getting - one that's set up to treat HC, CO and NOx. Both of the factory cats are needed to treat all 3 emitants - the first and second cats are different from each other.