I started off as a carpet cleaner and added window cleaning after about 10 years.
Carpet cleaning in a nut shell - It's a lot more complicated than window cleaning - you will need training before you start. The equipment is probably a lot more expensive too.
Most of your customers will be new customers - not repeat business and no "round".
I don't have rounds, so the carpet cleaning and window cleaning can work together, but they could give a problem when planning your work.