As far as our rules are concerned, it is ok to publish separately on Leanpub and Amazon at the same time. However, Amazon does have some restrictions you might want to look into if you haven't already. For example, if you use KDP Select, our understanding is they do not allow you to publish elsewhere at the same time. Also, they may have a rule that says you can't sell the book more cheaply elsewhere, even if you're not in KDP Select.
We have had authors publish on Leanpub, then "retire" their book on Leanpub so they can publish through Amazon KDP Select, and then take their book off Amazon and come back to Leanpub. We encourage experimenting with different ways of selling books, though obviously we're biased towards recommending Leanpub :)