Can I install 3-way switches using two 14/2 cables with another switch for a separate light?
The installer thought they could use 2 cables to get 4 conductors.  Not allowed per 300.3(B) due to AC magnetic effects (the same thing that makes transformers work that makes it even possible for the general public to have electric power.)
Use smart switches which use either wireless or powerline signaling, such as Intsteon... and then you can do this.

Black = always-hot
White = neutral
Red = switched-hot 1
Blue = switched-hot 2
You don't have to use colored tape to designate conductor by function, but it's cheap and it makes boxes a lot less confusing.