I think that can be a pareto set, but i imagine a utility function not conventional. If we compute the utility of individual one as U1(x1,x2) = (x1-3)^2 + (x2 - (x1-3)^(2/3))^2 … ( the equation of a heart) can represent the silhouette of a couple as the photo of David Lynch and Isabella Rossellini. The another utility, for be a pareto set in a edgeworth box, could be a conventional utility function with sustitutes U2(x1,x2) = x1 + x2
The assumptions that violated would be u-convexity. I have doubht if transitivity also, but definitly monoticity no, because the marginal utility for both goods are positive except in the middle point of the heart…
I add a picture of the first utility function because it reresents the silhouette of the photo, of course only consider the first cuadrant.