Suppose you want use the "F1" key for your "Fold+Stack" hotkey. Since you are using two instances, you need to ensure that both instances will recognize this keypress. So just type a ~ at the front of the keyname when you are entering it, and then press 'Add Hotkey'. So you would use "~F1" instead of "F1". I'm not sure if this needs to be done in both instances or not, but its probably safe to do it the same in both.
The ~ allows the key to be passed through to other programs. Usually when you set up an SaT hotkey, that key is captured by SaT and thats it. So if you have the letter "A" set to a hotkey action, then when SaT is running, if you tried to type in an email, you would notice that the letter "A" was blocked. Using the ~ would allow that "A" to still pass through to other programs as well.
The ~ allows the key to be passed through to other programs. Usually when you set up an SaT hotkey, that key is captured by SaT and thats it. So if you have the letter "A" set to a hotkey action, then when SaT is running, if you tried to type in an email, you would notice that the letter "A" was blocked. Using the ~ would allow that "A" to still pass through to other programs as well.