What GR Jensen said. I'd add: When you are working with green (wet, also unstable) wood the wood will move often a lot as it dries - it warps and gets wonky and out of kilter- So twice turning means turning green to the rough shape and thickness and then set it aside to dry and stabilize (stop moving) so you can finish turn a concentric, evenly formed project. You can do so with Spindles as well, but long spindles going out of round can get so wonky and out of shape you pretty much have nothing much left to turn once you get it back to round/straight - However spindles being turned as boxes, goblets , etc. can often be roughed out and then stabilized and final turned just like is commonly don with bowls.
On the flip side, you can once-turn green bowls, goblets etc with the express purpose of turning it to final thickness and letting it warp (which can lead to some very artistic pieces) and likewise with green wood to turn long thin spindles and then let them warp as they will (positioning the grain strategically to get things to warp the way you would like them to is a skill in itself!)
Bear in mind, even kiln dried or air dried wood can and will move as the humidity in the environment changes, but they generally do not move quite as much as they would if they had been turned green.