I’ve proved the following (for every funcoids $f$ and $g$):
Statement $\mathrm{up}\, (f \sqcap^{\mathsf{FCD}} g) \subseteq \bigcup \{ \mathrm{up}\, (F \sqcap^{\mathsf{FCD}} G) \mid F \in \mathrm{up}\, f, G \in \mathrm{up}\, g \}$ or equivalently: If $Z\in\mathrm{up}\, (f \sqcap^{\mathsf{FCD}} g)$ then there exists $F \in \mathrm{up}\, f$, $G \in \mathrm{up}\, g$ such that $Z\in\mathrm{up}\, (F \sqcap^{\mathsf{FCD}} G)$.