Spring - coil spring confusion needing diffusion!

some idea has come to mind that maybe two springs can work out the same one longer but not so stiff the other short but stiffer (see attached pic)

As you can see the same make of spring has common oem numbers but are different for the same car model and type (5002.LK). The online shop has listed them both for my car.

I'm not sure which to choose as I do not know the oem for my car. Can this be determined from the vin number somehow online?

now according to this petrols and diesels have different springs

Found on the late model 1.6 HDi 92 (DV6ATED4) diesel

5002.LK (Quantity 02);

- WITHOUT AIR CONDITIONING UNTIL RPO 10660
REFERENCE: 1 YELLOW + 2 BLUE

- FIXED ROOF UNTIL RPO 10660
REFERENCE: 1 YELLOW + 2 BLUE

If thats correct wherever that came from then mine are 5002.LK according to those colours and I think my citroen is one of the last made late in 2009 so that makes sense.

...only they both list 5002.LK but are different heights

They are different rates. By 'rate' I mean stiffer or softer. The spring on the left (long and thin) is a softer spring. The one on the right (short and fat) is a stiffer spring. You would chose a spring depending on whether you want comfort or performance or light or heavy loads.
The ad leads you to believe the springs are a replacement for a particular spring...they aren't. It's only the spring manufacturers recommendation. The different PSA (Citroen) part numbers will represent different rates of springs (they may also represent different manufacturer of the same spring rate...confusing). So its not possible to replace a number of spring RATES with just one.
What the ad is saying is these springs will 'fit' where any of those part numbers would be used but they only do two springs, stiff and soft, so they 'recommend' those springs replace those part numbers.
It looks like 5002.LK is right in the middle of hard and soft, so they recommend either springs.
The ONLY way to GUARANTEE you get exactly the right spring is to go to Citroen.
If you don't care that it's exactly the right spring, chose hard or soft, buy two, fit them to your car and move on.