What you want is certainly possible on one itinary. You just have to add the stopover in AMS to it.

It is also possible to do it on two separate tickets, and even check your luggage through. That is called nesting tickets. The airlines are not always very keen on nested tickets, because it is a possibility to cheat the fare system. I've done it without problems though, when I added other nested reservations later, simply because I had to do some additional travel. They can't forbid that. you see the grey zone is large!

In some cases everything on one itinary can be cheaper, too. So it may pay off to compare.