Technically yes, but you'll need to be careful not to overload the input of whatever device you are sending to from the LS-4 in this configuration; it could be potentially damaging to the input stage of the receiving device if it receives too many signals at once.

In other words, think of it more as a selector than a merge box. It would be perfectly safe to use it to choose one sound source (like a keyboard) at a time, but don't send it signals from multiple devices at once.