They're not digital signals. 3.5mm auxillary is an analog signal which is precisely the reason we're talking about ohmage, amperage, and voltage. If it were digital then we'd be talking about bits but we're talking about physical properties. I'm not sure of the operation of a splitter but I wouldn't assume that it splits them in half evenly. I think it would depend on what either the speaker demanded and what the headphones demanded.
For instance, current is not a fan of resistance and will always follow the path with the least resistance, i.e. whatever path has more resistance will receive less current. Voltage, likewise, will be split but it would be a factor of that particular resistance x whatever current is flowing through it (ohm's law). If both Headphone and Speaker had the same impedance, your assumption would be right, but I doubt that's the case.