What do you hope to gain by trying to run 2 20A lines into your 30A cord? The 30A cord only uses one hot line so you will still be limited to 20A of power coming in.
That's not quite accurate, Alan. Two 20A hots could be fed to the single 30A hot if the right conditions are met, i.e. that both are of the same "phase". And for practical purposes, they need to be independent 20A sources, not just two outlets on the same circuit. That would put the two 20A sources in parallel and probably allow 30 amps to be pulled.
WA8yxm cited some other practical considerations that could result in less than 30A being available too. All in all, this is not an practical approach unless you have in-depth knowledge of the 20A source circuits.
I have actually done this, but it was at my brother-in-law's house where he & I chose two suitable circuits and used them to power my 50A coach for several days. I used my 50/30 cheater box with two 30/20 adapters to connect to the 20A outlets.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL