Let's start with terminology. Don't abbreviate abbreviations, especially if you don't even understand them. It's High impedance or floating or tri-state or high-Z or hi-Z or hi Z, but not HIZ. (Z is the traditional math symbol for impedance, but that's not clear if it's not its own word.)
Neutral? If you're referring to high impedance, that's the wrong word entirely. Floating is not neutral, it's indeterminate. If you mean high or low, also wrong. If you mean default, default state is what pull-up resistors are used for. Bottom line, neutral is only a good electronics word if you define it before using it.
Yes, SDA and SDL are signal lines, each of which may need a pull-up, but still only one bus (a group of signal lines).
Yes, there may often be multiple pull-up resistors on the bus, but in your case the distances are so short that there's not much point. The wires aren't long enough to have much resistance, and all the devices are using the same power source.