We have sprints with a lot of items, several people are working on.
Sometimes, dev1 has to wait for dev2's finished item before he can finish and test his  own work.

Certainly, dev1 could work on other stuff in the waiting time, but here, Zoho Sprints forces a break of the idea of agile sprints.

I can not find how to bring all the items into an order to have a smooth sprint.

How are you dealing with this?