Just need to confirm my thinking:::

[andylo]

If I am stacking 2 x flashlights together...

I did NOT extended it's effective distance.

I ONLY double amount of light illuminate to my subject. (Which means my subject is 2 x brighter)

Looking forward to you answer :)

[Wayne]

Yes, assuming both lights are =

[andylo]

Yep, same setting everything. Same model of light too.

[ameerat42]

And, if they are of equal output, your exposure is increased by only 1 f-stop, though... Am.

[andylo]

Oh! really? Damn, I thought it's more than that :(

[ameerat42]

Nope! N times the light/area of glass/aperture = sqrt of N increase in exposure. (This is the BRIGHT side, if you had 3 equival flashes you'd only 1.7 times the exposure. So you'd need 4 equiv flashes to get an increase of 2 f-stops.) Inverse square law for light intensity.
Am.

[Longshots]

very useful response Ameerat42 :)