I agree with comments above and add that the current one I have uses 25 feet cords on each end which is usually overkill. Others have used 10-15 feet cords and done fine with them since most of the time you will be able to find outlets on different phases relatively near each other. (The closest that I've found is right next to each other! but this is not common).
The advice to match the receptacle to the capacity of the device is a sensible one; this is what I do by using a L6-20R. Although I do know people who use unmatched receptacles. More recently a friend of mine used a 5-20R for his receptacle which is, of course, amperage matched but not voltage match since 5-20R is 120V. Still, I think it's much safer than using a Y type connector without fuses. I just found out that EMW sells Y connectors with 2X120V to 14-50R which is not only unmatched but also without the safety relays as far as I can see. This is from EMW's website
(not for the faint hearted):
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Dual 'regular' 3-prong household plug - to convert 2 120V outlets in your house into a mighty 240v supply. You will have to provide your own extension cords to reach those two sockets (which will have to be on different 'branches' in your house wiring). You can get those extension cords pretty much anywhere (Home Depot in the US, etc). Just make sure you use heavy duty 12 or 10 gauge cords. With your purchase of this adapter, we will be supplying directions on how to identify which sockets sit on different branches. It is really easy. You can also test your circuit yourself before ordering to make sure it will work with this adapter. Note the breakers for your respective outlets that you would like to use, then in your electrical panel, measure the voltage between the wires that enter the respective breakers. You will see either 240V or 0V. If you see 240V, you can use those outlets to get 240V. Note that the outlets have to be regular 120V outlets, not the GFCI type (GFCI outlets normally have a test button and a small indication light)."