Info on dual station temp gauges / senders

[reel_crazy]

I have a tower on my boat and have 2 sets of temperature gauges. I want to know what is the industry standard spec for this setup. The gauges appear to be 240 ohms. Should they be wired in parallel or series? From looking at parts catalogues they specify senders for single or dual gauge setups , what is the standard for the water temp sender, i.e. ohms for a single station sender versus dual.
thanks
rich