My understanding is

y1 = x1+x2

y2 = x1+x2+y1 y1 is endogenous variable of y2 (does it describe the causal effect too? i.e. in the joint decision process y1 decision is taken first in the order)

y1 = x1+x2

y2 = x1+x2 Can we call this simultaneous equation? considering we are using copulas i.e. both of the decisions are taken jointly without any order

y1,y2 are response variables. i avoided writing the whole utility equation to keep it simple

Kindly correct me if i am wrong.