Oh dear.
It's not 'cruel keep domestic cats (aka the pet cat) safe indoors, in the same way it is not cruel to keep a dog under constant supervision, a rabbit in a hutch, a cow in a field, and any other animal that is not wild.
All of these constraints could be cruel if they were abusive, like battery hens or not exercising pets etc.
Look up the word 'domestic'.
Ok now, consider why no one lets any other pets, like dogs, out unsupervised. Dogs hunt, so surely its unfair to stop them killing things by that argument?
Then lets consider what makes a pet happy. I think avoiding harm is quite high up that list.
Then Google information on the impact of domestic cats on local wildlife populations. What's more cruel - preventing a cat from killing or letting mice, voles, rabbits, birds etc die a slow and painful death at the paws of millions of cats?
Op, taking in two cats sounds lovely, good luck to you. Please the ignorant comments.