If you accurately divide the fiber in half, and manage to have the same amount of waste (neps, VM, snags by the cats...) for both spindle-fulls, and manage to spin exactly the same "grist" (think, thickness/ compactness/ wraps-per-inch-ness), then you might find each spindle has exactly the same length of fiber on it. Ditto bobbins on a wheel, for that matter.
Buuut, if you are not so exact, like little-ole inexact me, then you will be left over with a bit more or not so much from one of the spindles when you are plying. What to do with the left over bit? The Andean plying bracelet will let you exactly two-ply the remaining bit. Take the bitter end that finished and roll it together with the loose end of the Andean plying bracelet, then continue plying on from the bracelet until you reach the "end", which is a loop, with nothing left over.
Now, if you are three-plying ... well, here's what I did on my wheel (yes, I hate to waste anything!) When the first bobbin ran out, I took the less-full of the two remaining bobbins and Andean-bracelet-ed it, using its 2-ply and the other bobbin to continue 3-plying. Then when that ran out (this is why I picked the emptier one ... zero waste), I Navajo-plied (aka chain plying) the remaining yarn on the last bobbin for a 3-ply (granted with the little chain-bumps) right to the very end.