The only risk is that the upside down tank method will easily overfill that 3.5oz. So just make sure you weight it before you hook it up, and then again after you unhook it.
Instead of on/offs, a simple solution is a 90-degree bend. Connect to ASAs together, so that they are at a 90-degree angle.
Connect the 3.5oz, then the 20oz. Tip the 20oz upside down to fill the 3.5oz. Then flip the 20oz so that it makes a "^" shape. This will settle the liquid in the 20oz and 3.5oz
Then remove the 3.5oz enough just until the pin is unpressed, but oring is still sealing. Then remove the 20oz all the way, then the 3.5oz all the weight. (Weight and drain the 3.5oz).
This was the method we used in the old days. Sure, remotes and on/offs are handy, but sometimes you just don't have them.
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