Ultra Sun Dual Arc Tube -- 1000 Watt  These are discontinued now.

The Ultra Sun® Dual Arc Tube Lamp offers the best of both worlds with a 600 watt HPS and a 400 watt MH arc tube both inside one lamp jacket, operating simultaneously. This full spectrum lamp has a 3,000° K color temperature achieved by the dual arc tubes working together. This lamp is an excellent choice for full-term growth, flowering and fruiting. The Dual Arc Tube lamp operates on a standard 1000 watt HPS magnetic ballast or an electronic ballast such as the Galaxy® brand. These lamps have been specifically engineered for horticulture applications. Ultra Sun® lamps are high quality, feature robust construction and have excellent reliability at an economical price.

There are sodium lamps with 2 arc tubes wired in parallel.This is good for streetlighting.What is the main purpouse of this lamps ?

There are sodium lamps with 2 arc tubes wired in parallel.This is good for streetlighting.What is the main purpouse of this lamps ?

To irradiate plant both the spectrum of a HPS lamp, and the spectrum of a daylight rare-earth MH lamp from a single lamp, instead of using two separate lamps.

"This lamp is an excellent choice for full-term growth, flowering and fruiting"

Thanks for answer.So lamp lasts longer and reduces maintenance costs.

Thanks for answer.So lamp lasts longer and reduces maintenance costs.

You are talking about a different lamp than the one in this thread: The twinarc HPS lamp which your question is true there. In the lamp at this thread, both the HPS and the MH arctubes are connected in series, so both lights at the same time, so your question is false in this case.

Because it does not make much sense cost wise.
This lamp is very expensive due to being a niche, so not that much cost savings against usng two separate lamps.
Each arctube has to operate at low voltage, they are each way less efficient than the dedicated HPS or MH lamp. So the running costs become higher than of the two separate lamps.
Because large industrial growing needs many lamps anyway, there is not much benefit of having e.g. 200 of such combined lamps instead of 100 HPS and 100 MHs, in either case it is 200 fixtures. Having the HPS electrically compatible with MH (or the use of ballasts capable to feed both types) makes way greater sense there - replacing the lamps as the plants grow to different phases becomes easy. Many installations have just both types installed with higher numbers and the changes are done electrically by switching more of MHs iso HPS (or vice versa). Any case, this needs separate lamps.
So the only use becomes some small pot or so. But that market is very tiny, it just does not generate that much sales.
And the "small pot" business was the first, where these discharges became threatened by LEDs, reducing the sales further.
So much, manufacturing them stopped making financial sense...