2x4ah would just be an 8ah wouldn't it?
Ie, the 2x 5ah will still give you a longer total run time
yes., but.....
the batteries have C rate (I think it was mentioned earlier)
*normally* they measure the Ah capacity while draining it at 10% of capacity, so drain a 4ah battery at 0.4amps and a 5ah at 0.5amps.... see how long it lasts and that's your Ah capacity
but... if you drain it slower it lasts longer and if you drain it faster it doesen't last as long.... because of the efficiency in the battery chemistry turning "chemicals into electric" (short version)
there's also a bit of recovery time, where it takes time to convert "chemicals to electric" - if you pull power out faster than it the battery can keep up the battery will run out of power quickly... but then recover a bit later on when you're not using it
it's a bit like starting an old car on a cold morning, you crank and crank, you can feel it slowing down and the battery start to die... so you stop for a moment and give it a rest.... suddenly it's cranking fast again! (as the battery recovers a bit)
if you're draining them fast/hard you'll lose a lot of potential capacity
if you drain them slow (say half as fast because there's two of them) they last a lot longer