You've got a couple of different things mixed in your head, I think. Which is excusable as the decades pass. At any rate, the maximum current draw is just plain old linear per cell, so two 5A cells in parallel will deliver 10A. The current-squared thing has to do with power dissipation in a resistor.
Just as a side note, alkaline cells are so much better at delivering current than plain old carbon-zinc cells that a pair of "D" cell batteries in parallel will light up an engine better than a nicad glow-starter, and they'll last forever, too. I put banana plugs on the end of my glow-starting lead, and use the magic size of brass tube (I think it's 5/32", but I won't swear to that) into which they fit. I just solder two batteries together in parallel with lengths of the tube, wrap it in electrical tape, and tra-la, I have a battery.