If you fill both tanks from the bottom, regardless of size, both tanks will rise together, from the same level from the ground. In other words, if one tank's bottom is six inches closer to the ground than the other, that lower tank will fill up to the point where it is even with the bottom of the other, then the two will rise together. Water seeks it's own level. HOWEVER, you have to let air come out of the second tank. If you are filling it from the bottom, the air HAS to be vented out the top.
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