I think the difficulty using Lightning with Rasengan... is that the lightning has condensed and controlled WITHIN Rasengan... Considering that Lightning travels faster than the rasengan spins... lightning will MOST LIKELY never be able to be contained within it.hm..lets see.
chidori and frs are similar jutsus.in both case you must concentrate your chakra in your hand and in both cases the chakra is visible!
so....
rasengan+wind typed manipulation=frs
rasengan+lightning typed manipulation=chidori?
(kakashi can make rasengan as we know and he was tought it by his sensei minato so its quite possible!)
I can see it working like this....
2 Naruto's with Rasengan's... 1 rasengan = Negative lighting and 1 = Positive... If Naruto hits 1 person with the rasengan, and releases the 2nd... the second will go flying into the person he hit with the first... double the lightning and rasengan attack.