I agree with everyone else, get them both altered at the same time. You were given some very good reasons for altering both the male and female, and I agree with them all.

I also agree that the problem won't be recovery time, it will be how to convince them that they NEED recovery time. 24 hours after a spay is the longest I've ever managed to keep a dog "calm and quiet". You will be amazed at how quickly they bounce back.