You can start using both at the same time.
If I could turn back the hands of time, I would start the Little Math first. From my experience, Little Math is a very challenging program to teach. It's a hard core flash method program. Young babies ,who are less mobile, may find this program "fun" or "interesting". However, when My toddler started moving around or walking, he didn't care for Little Math. Compared to Little Reader, Little Math's contents have less action or should I say "interaction'?, you know. It's pure flashing dots or icons. Surely, it helps improve your child number sense and math concepts but the challenge to keep your little one interested in Little Math is.......like climbing Mount Everest..
Having said that, we did manage to complete the curriculum plus the Shichida one.
So, start with Little math, and after 4 weeks or so, you may start the Little Reader software. Little Reader is a lot more fun and engaging. You can interact with your child by playing, chatting, or using your own props to make the lessons much more fun.
Schedule wise, you can do both in 1 sitting. The daily lesson for both program are divided in 2 sessions. During the first sitting, show Little Math Session 1 + Little Reader Session 1. Take a break, and then show Little Math Session 2 + Little Reader Session 2.
That what I had done and it worked pretty well for us. Don't wait too long for Little Math. Start now when your little one is young!