2 plants one big pot

I have already try it with photoperiod seeds for "science purpose" I ended up with same yield for this 2 plants together than each other plants of same strain

No, they will compete for root room, nutrients and water. Instead of getting two plants, you end up with 2 half plants. Have tried this before and am experienced with the disappointment growing like that brings! In reality, one plant, one pot is the only way to grow. BTW, 20 litres is what I would consider as the minimum for one plant on its own.

And btw, a 20L pot is very big.... I've grown autos (just ONE of them) in that size and usually end up being unable to separate the root ball/soil from the pot at the end of the grow.

One pot, one plant. Period.

I have done this multiple times and I don't recommend it. Two main reasons - first is that one plant will ALWAYS dominate the pot. That will ultimately leave you with one tiny plant and one massive plant. Second is that the two plants will compete for nutrients and you'll notice that one plant has deficiencies that the other does not - this is particularly apparent when you use two different strains. This is hard to fix as anything you use to try and remedy it will also be absorbed by the healthy plant. As UnkleSkunk says, it is technically possible to do, it just requires more effort... and as Ezzjaybruh says, that effort is just totally not worth it. Take it from someone who has done what you're asking many times (using two autos in a 30L pot)... just get another pot, you'll get better results with the same amount of time and very little effort! 😜

Can you? Yes. Should you just do separate pots? Yes

riscrivo avevo letto male.... si lo puoi fare i problemi sono l'incrocio delle radici una pianta potrebbe prendere il sopravvento e sovrastare l'altra io ti consiglierei di dividere il vaso in 2 con un pezzo di plastica cosi da avere un vaso con 2 spazi per la radicazione separati e di fare un lst per distanziare la parte superiore