Can I let my indoor cats go outdoors?

No they wouldn't be ok. stray cats carry disease that there are no vaccines for and your cats could fall prey to. like leukemia and feline aids. do you want your cats to get into fights or get run over? they will not steer clear of strays they will head toward a fight and you will be responsible for their deaths. Don't let them out. You are just feeling some wierd sense of guilt but don't . you took these two cats to protect and care for them don't just throw them out into a world they don't know and watch them die.