If you're going to carry both as backpacks, put the aeronaut on your back first, then put your synapse on "backward" so that it's actually on your chest, rather than your back. It will take a very small effort to keep the synapse from sliding off, but just keep your shoulders wide and not slumping forward, and you should be fine.
For moderate distances, you can use the absolute strap to carry you aeronaut with the strap over one shoulder and diagonally across your back like a messenger bag that you're getting something out of. If you do that again put the aeronaut on first, then the synapse. Also, if you do that, you may want to shorten up the absolute strap so that the aeronaut is mostly in front of your torso, rather than banging into you legs with every stride. You might be able to get away with doing somewhat of the opposite and having the strap a little longer so that the aeronaut hangs to your side and below the backpack. I wouldn't do that for too long a distance unless the aeronaut is lightly filled--much more than getting into or out of an airport--though because the aeronaut will definitely affect your stride.
And make sure to practice at home so you know how long to have the strap in advance.
