Abnormals are written for one failure/caution/warning only - i.e. this procedure assumes two engines running. L+R WING TK LO LVL happens at approx 30 mins. fuel remaining.. I would hazard a guess that closing the x-feed is requested to prevent simultaneous flame-out.
I would keep the fuel x-feed open in your scenario with one engine shut-down - it is one of those situations where common sense must prevail. And you are not going wrong - you are displaying good technical understanding.