I don’t know why the “Y” connector would be considered dangerous. You may be thinking of what some do which is make a cord with male connectors on both sides of the cord and plug it into a generator and a home on the other end. This is indeed dangerous as you would have have exposed electrical contacts on one or the other end of the cord which may contact a person resulting in injury or death. But…this is not the “Y” cord mentioned.
I did not come to he conclusion that the “Y” connection was definitely dangerous, but only was reporting that some people contend that it may be or is. I haven’t come any firm conclusions on this issue.
It is my understanding that all the AC outlets are connected and share the same circuit. If that is true, combining two outlets will perform no differently than one.
Then what is the point of this combiner? 
To use at an rv park where you can plug into to completely separate 120 volt circuits to get double amperage. If you plugged that thing into both ac outlets of an AC30, you would still only have 300 watts of available power
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