Generally speaking, most cat backpacks are designed for one small cat or for medium-sized cats. However, if the cats are very young and the cat backpack is large enough, there may sometimes be room for two cats. That being said, large cats should never double up in any space.
Similarly, cats who don't get on should never be placed into the same space, since this will simply add to your stress and make travel unbearable for your cats. At the same time, you should also consider the added weight of more than one cat at any time. Not only can this be bad for your back - but it can also increase the risk of the pet backpack struggling to hold the additional weight.
As a general rule of thumb, if you need to bring along two cats, then you would be better suited to a carrier, rather than a backpack. As always, consider the health and safety of your cat before making any decisions and work with what's best for their personal needs, as well as your own.