Oh, so those guys are doing theater here, too? You can be forgiven if you thought Patrick Stewart and Ian McKellen simply were wandering the boroughs, posting selfies and behaving like rascally old coots, albeit coots who’ve been knighted by Queen Elizabeth.

The pair have been living it up for months, Stewart Tweeting images of the duo tossing back ale at McSorley’s, McKellen posting photos of the BFFs “jogging” over the Brooklyn Bridge. But yes, Xavier and Magneto also can be seen live, on stage, performing two absurdist plays in repertory, “No Man’s Land” and “Waiting for Godot.” Both have just opened at the Cort Theatre.

Though written decades apart, the connecting tissue between “No Man’s Land” and “Waiting for Godot” is that their central characters are trapped, yet don’t do anything to escape their situations. The two pieces are explorations of aging and friendship, and the unreliability of memory. If you can possibly relate to such notions -- you can, of course -- then Stewart and McKellen make for trustworthy guides through the murky postmodern territory carved out by Harold Pinter and Samuel Beckett, respectively.

Neither is an “easy” play. The plotless narratives leave audiences to do much of the work; the former is dark and confusing, the latter existential and at times despairing. But there’s something so gee-whiz trustworthy about Stewart and McKellen, sonorous-voiced men if ever two existed, that you’ll gladly go along for the fearsome ride. Director Sean Mathias, who also guided the pair in “Godot” during a 2009 run in London, has said Pinter’s play is about trying to make order of things, while “Godot” is about “creating chaos.” Both sentiments ring true after an all-in-one-day viewing (each is performed four consecutive days before switching to the other, with both performed on matinee days).

In the staid “No Man’s Land,” Hirst (Stewart), a successful poet, and Spooner (McKellen), a failed one, having met in a London pub, drink into the night in Hirst’s well-appointed home. The overtly homosexual overtones in some productions -- Hirst and Spooner meet in Hampstead Heath, known as a gay-cruising destination -- are not particularly dwelled upon here.

What we are meant to wonder about is whether the two writers, Hirst dapper, Spooner a real shlump, really know each other, or if perhaps, as has been suggested, they represent alternative fates of the same man. The delight here is in moments as base as watching Stewart gradually descend into stumbling drunkenness -- Captain Picard? Wasted?! -- and then return the next morning, in the second act, remembering Spooner as an Oxbridge chum, but not as the previous night’s companion.

Matters get more confusing with the arrival of two other men, Hirst’s menacing live-in aides, Briggs (Shuler Hensley) and Foster (Billy Crudup), who may also be lovers. They’re suspicious of Spooner, though we never find out why. McKellen has the showiest part, as he moves from caddish opportunist to cornered prey. Pinter is not for everyone, and if you’re the kind of person who likes your theater tied up in a bow, you’re likely to get lost in “No Man’s Land” … though with Stewart and McKellen at the wheel, at least you’ll be making good time.

“Waiting for Godot,” to my taste the more palatable of the two plays, has the famous wanderers Vladimir (Stewart) and Estragon (McKellen) idling by a barren tree, expecting to meet up with Mr. Godot, who they hope will improve their lot. Beckett’s play is an examination of time, memory and identity, set, we’re told, over consecutive days.

With their bowler hats and Chaplin-esque tramp movements, Stewart and McKellen come off as a pair of frustrated vaudevillians vacillating between feelings of gleefulness and suicidal ideation. Stewart’s "Didi," as he's nicknamed, is the more humane and hopeful of the pair. Though undermined by disease, he takes care of McKellen's "Gogo," helping out when Gogo is kicked by Lucky (Crudup, sublime in a role that essentially calls for him to just drool), the nearly mute slave to the character Pozzo (Hensley, clownish and compelling). McKellen’s Gogo is elastic, cranky and mischievous, complaining about his aching, stinky feet, or poking at Lucky to try and make him dance.

Stewart and McKellen traverse the full range of humanity on stage, with moments of hope, fear and despair. They leave you feeling nothing less than the unbearable lightness of being. As my companion opined exiting the theater: “Your feet hurt, and you might hang yourself tomorrow, but there’s still a bit of carrot left, and look, a leaf has popped out of the tree. Maybe it’s spring?” Heck, who can't relate to that?

McKellen and Stewart's joyous pairing is a once-in-a-lifetime opportunity to see and hear these two knights of the theater clown and caper and kvetch their way through what's been called the most significant play in the English language. Hearing the voices of those familiar, classically trained avatars of authority (“Thou shalt not pass!” “Make it so!”) in person, tackling questions of our human existence, just isn't an experience you'll ever get on Twitter.

“No Man’s Land” and “Waiting for Godot,” through March 2, 2014, at the Cort Theatre, 138 W. 48th St. Tickets: $40-$137. Call 212-239-6200 or visit Telecharge.com.

Follow Robert Kahn on Twitter@RobertKahn