Oh yes, please do get them spayed. As soon as possible. Constant heat cycles every 3 weeks will drive them (and your household) crazy. Spaying also prevents diseases such as pyometra (a deadly uterine infection) and uterine cancer, and reduces the risk of breast cancer.
Also, eventually they WILL get out and get pregnant.
So get right on that!
But I have some questions, first about their litter boxes.
Are both cats avoiding the litter box, and both pee and poop?
How many litter boxes do you have? How often are the litter boxes cleaned?
Where are the litter boxes kept?
And now about their level of socialization:
How long have you had the cats?
What kind of interaction with people do the cats get? Does any one play with them? How often?
Is your household busy and loud? Or quiet with little activity and no people ever coming in?