Two line for one air stone?

"Can I use two air pump lines on one air stone in for dwc ... " -- sure.. additional resistance could create more wear and tear on teh pump and reduce longevity, of course. So, i'd look at how the current stone is performing and if it is optimal, i wouldn't try to force more air through it. If it could use a bump in bubble production, it's a great idea. would running a 2nd airstone in each bucket be better? if it pushes teh first one fine, just add a another stone if you want more bubbles. could get a little manifold and close off unused lines, too. allows for expansion later. some more utility than a plain T - can adjust flow of each line to even it out if needed too. just an option to throw out there. The second part of your question sounds different.. 2 buckets 4 outlets? sounds like you want to run 2 lines to each bucket with 2 airstones at end.. vs. 2 lines to 1 airstone in each bucket. either way.. just use common sense based on the current function of your airstones. Remember, gases dissolve into liquids almost instantly... it takes very little effort to oxygenate water. churning is actually more efficent than bubbles, this is why you see wave makers in larger aquariums. constant rotating of water at surface -> constantly dissolving gases and mixing it up as it flows. you don't need that in a bucket.. .but proof of concept of what i just explained. don't worry too much about this... if it has 'enough' dissolved o2, mission accomplished. small bubbles will oxygenate well, larger bubbles will stir/agitate better to avoid stagnation / biological buidlup on sides etc... if you do go 2 stones per bucket, maybe get one with fine bubbles and one with large bubbles? best of both worlds.