what you should do, is get a used, but still good, lamp adapter that you can hack, then split the single power line into 2 lines and use that.
some of the duplex electrical outputs have one 120V phase of the 240V on one of the outputs and the other phase on the other output so that a 15A toaster and a 15A kettle can both be used at the same time. The live wires from each outlet produce 240V like the stove and clothes dryer use.
What is being talked about here is a horrible idea and a good way to get someone hurt or killed.
If one wants a 240v outlet, just have one installed properly for god sake.
By suggesting that one can plug into separate 120v outlets to derive a 240v circuit, you would end up with live exposed plug prongs. If either plug was removed, someone would likely get electrocuted because power would be feed through the load and out to the other plug.
Splitting up power on separate lines causes hysteresis heating in cabling behind walls.
Also, If there is a short circuit, only one breaker will trip and power will not be totally shut off to the circuit.
All 240v circuits are required to disconnect both phases at the same time for this reason.
The breakers are also required to be dual 120/240v rated for a 240v circuit as opposed to single 120v rated breakers.
Anyone who thinks hacking into household mains or modifying a ul listed cord is a good idea needs to think again.