Gharbad
Well I used to burrow multiple units in the same spot.
Then when they come out it sort of bugs out, and they're all on top of eachother. I think they also end up firing or something if an enemy is within range, I don't remember.
Well that, for a fact, doesn't work b/c if u burrow them all in the same spot, they will just spread out and not be stacked at all.
But if u want to learn how, heres how u do it: NOTICE: This is the same strategy used to stack lings and drones and any other burrowable unit.
Step 1: Select a BURROWED hydra and HOTKEY it.
(ctrl+0-9. usually ctrl+1 is the easiest)
Step 2: Now select your hydras that are UNBURROWED and quickly press the following: (should take less than a second) U, ctrl+1, 1, U in that order. Don't go too fast or u'll mess up. Try to do it at a mild speed. Not too fast, not too slow.
Step 3: NOTICE: If you click a hydra when its stacked before you move it, it should still look burrowed but the green area outside (you know, where you select your unit and it has a little green circle around it) is big instead of small. That indicates that you have stacked correctly.
NOTICE: DO NOT BURROW WHEN HYDRAS ARE STACKED. IF YOU HAVE A MASSIVE AMOUNT OF HYDRAS, YOU WILL HAVE TO RESTACK THEM ALL OVER AGAIN.
Step 4: If done correctly, hydras should appear burrowed but you can move them around. Select the ones that are stacked (if they are not, simply just restack them) and move them to a designated area. Once they are all in one spot, hold position. That way they dont spread out and attack an enemy that is running off.
Step 5: Thats it. There is my tutorial on how to stack hydras.
I may be able to show you what stacked hydras look like if i can get a picture.