Тогда я бы так написала
There are four main combinations of X as a bottleneck and Y as a non-bottleneck. They demonstrate that the whole system can only be determined by the bottleneck irrespective of the direction, in which the parts flow.
It doesn't matter whether the parts flow from X to Y or vice versa, in both cases it is important to produce not more than can be sold. Besides you must take into consideration the fact that X is a bottleneck, i.e. it doesn't have extra capacity and you need all 100% capacity of the X machine to meet the demand.
In case the products are worked on separately (one product flows through the bottleneck, the other does not) but at the end of the production proсess combine into a single product, we must be oriented on demand and the bottleneck capacity.
If Y produces more than can be sold it means that the stock is being built up
In the other case √ the products are produced separately and are sold separately as well √ it seems at first glance that we can use both machines to 100% but in case of Y we should take into consideration the market demand. So the level of utilization of a non-bottleneck is determined not by its own potential but by some other constraint in the system.
P.S. Но фраза "produces more than can be sold" остается неправильной, т.к. нет дополнения-подлежащего, которое "can be sold". О каком производстве речь?