Are Barlow lenses stackable for bright objects? I'm back to the refractor that I own, and I want to maximize the zoom capacity. I realized today that I own more than one barlow lens, and I was wondering if I could stack them together for looking at bright objects, like Jupiter. I'm aware that looking at anything else would probably be disastrous due to the stacked barlow lenses taking up more light. Is this an effective, or desirable way to approach the situation? What will happen if I do stack more than one barlow lens? Will it have the effect of an enhanced image, or will it do nothing but remove light?
Yes, stacking Barlow lenses is a common practice to effectively increase focal length by multiplying their individual focal lengths. When I say common, most advanced eyepieces actually have many glass elements and are a type of a Barlow lens themselves, so just by using a single Barlow lens in front of your eyepiece you'd already be, technically, stacking Barlows. It is a technique that is particularly useful for bright object like our Solar system's planets, with Jupiter that you mention perhaps even the best example.

Your image quality might vary substantially tho with the Barlow and eyepiece quality of glass elements and the manufacturing precision. You might also want to play with the order at which you stack them and find an optimal arrangement of all of the glass elements they add to the system. The more precisely they're made, the less the order will matter though. And you'd technically want the less precise ones at the end of the stack, so their deficiencies don't magnify with each added element. And yes, with any additional glass element you'll be losing a bit of the illumination so this is, like mentioned, most suitable for bright objects.

Do not however, under any circumstances, do this when observing the Sun! You shouldn't need additional glass elements anyway, but should you try, you'll most certainly cause them to overheat, expand and permanently deform quite fast, regardless what material their tube and thread are made of, metal, alloy or plastic.