One of the more fully formed portraits of life during lockdown, Stephen Daldry and Dennis Kelly’s rapid-fire, fourth-wall-breaking, two-hander “Together” trades the overt sentimentalism of Daldry’s previous film work—“The Reader,” “Extremely Loud and Incredibly Close,” etc.—for a darkly comedic study in the ups and downs of a single couple as they contend with the fracturing of their relationship amidst the pandemic.