I tend to think of them as separate when studying them, however when thinking about composition you can think of them together, and might even sacrifice the accuracy of one for the other. For instance, you might bend or break perspective rules on a form to get a more pleasing shape or composition. But when studying form, it helps to isolate it. It helps to really nail down drawing boxes and cylinders from your imagination in any position since those are the building blocks for everything.

Hey there! Combining the two together seems like the logical thing to do to me. If you are doing an observational study you still have to take shape design into consideration. For example, if there is a weird overlap in the reference you should change the shapes so you eliminate the tangents. In general, I don't think form and shape can be separated from each other since they are kind of complementary, so if your shapes are not good-looking your form will end up looking wanky.

In my opinion, form (3d) comes first, being closest to observing the real world, and shape (2d) is how things end up on your paper. Start with form study, and when you are a little more comfortable with observing/manipulating/drawing things in 3d space, pay attention to how things look on paper.