if you want the flood light switched with the attic light, this could be relatively easy.
However, your left with two options here. either leave attic light on while flood is on(waste electricity)
or operate attic light solely with pull chain. (no attic light when switch is on)
neither of these are ideal, but it's your house
to do this, run 14/2 from the attic light box to the new light box mounted in soffit.
preferably you would want separate control over both lights.
for this to happen you'll need to feed another 14/2 from the switch box to the flood light.
if you dont want to expand a single switch box for a double, you can get a double switch that operates two devices.
like this one