unfortunately, its not that simple. First of all, two 80mm radiators dont even come close to 1 160mm radiator in terms of area, or flow.

80mm radiator surface area is 80x80 or 64 cm2 x2 = 128 cm2

160mm radiator surface area is 160x160 or 256 cm2

so in fact a a 160mm radiator is the same as four 80mms in terms of surface area.

If surface area was the only concern, then a 113mm radiator would have the same surface area as two 80mms.

It gets worse. Surface area isn't what we're worried about here as much as flow. In order to get the same amount of turns into a radiator of 80mm, you need smaller tubing and tighter turns than a 120mm radiator (as that is the next size up, slightly larger than 2x80s) you also need double the number of turns and twice as long in the tighter tubing since you have two radiators versus one.

The only thing I can suggest is to find someone who has tried a system with 1x80, 2x80 and 1x120 and has comparisons. Thats not going to be easy, but you can give it a shot.