By MEL GUSSOW

In ''The Lisbon Traviata,'' Terrence McNally has written the theatrical equivalent of an operatic double bill - an opera bouffe followed by a tragic denouement. The first act, in the play's newly revised version, is a savagely amusing and empathetic study of two men whose lives have been lost in opera. The second act is discordant, as it was in the play's earlier showcase production in 1985.

Since the play was first presented, it has gone through alterations, all of which are improvements and help to bridge the difference between the two acts. John Tillinger's new production, which opened last night at the Manhattan Theater Club, is even sharper than his original at Theater Off Park. The performances by Anthony Heald and, in particular, by Nathan Lane, are among the finest offered by these estimable actors. But the author has not solved the play's structural problems.

In the first act, we are sequestered in a richly baroque apartment - itself a kind of red plush opera box -with the host, Mendy (Mr. Lane), and his guest, Stephen (Mr. Heald). Both are addicted to the genius and the memory of Maria Callas, almost to the point of exclusion of all other singers, whom they dismiss with the most excoriating remarks. One does not have to be a music critic to appreciate Mr. McNally's wit and his encyclopedic knowledge of the art form under scrutiny.

When Mr. Heald mentions an obscure pirated recording of a Callas ''Traviata'' performed in Lisbon, Mr. Lane is swept away in anticipation. That recording, still unheard by him, becomes a kind of Holy Grail.

In the first version of the play, the dialogue between the two men was almost too funny. Though Mr. McNally still cannot resist ridiculing his artistic peers (past and present), the act has been reshaped so that laughter is underscored with portent. As the two reveal themselves through their references to opera and to their own relationship, we can sense the abject loneliness and the desperation of both characters. The first act is tragedy in the guise of comedy.

Mr. Lane, formerly married and the father of a son (as he says, ''Callas was named in my divorce for alienation of affections'') is, for all his campiness, filled with feelings of immense insecurity. He is, one might say, all show and no follow-through. In contrast, Mr. Heald's character is as obsessed by love as he is by opera, to the point of confusing the two.

The play is an ambitious attempt to confront demons absent or suppressed in the playwright's other, engaging work. Mr. McNally is taking himself and his subject with the utmost seriousness. For a long time, we are held by his acuity - and by the intensity of Mr. Tillinger's direction. The audience's anticipation at the end of the first act almost equals that of Mr. Lane awaiting the recording of ''The Lisbon Traviata.''

The second act moves to the apartment Mr. Heald shares with Dan Butler, an apartment that is the absolute opposite of Mr. Lane's - cleanly and stylishly modern (two splendid set designs by Philipp Jung). As is soon evident, the two halves of the play are as disparate as the settings.

In his home, Mr. Heald is unable to come to terms with what he considers to be Mr. Butler's betrayal. The latter is involved in a new liaison, which Mr. Heald, with carefully orchestrated malevolence, tries to torpedo. Though the confrontation retains the play's earlier vituperativeness, it swims into banal dramatic waters, as the homosexual characters echo cliches from fiction about heterosexual couples. Mr. Heald becomes the spurned ''wife'' deserted for a younger love and crazed in a pursuit of retribution. While the violent conclusion may seem inevitable in the context of opera, it is not convincing in this domestic drama.

The two minor roles, though deftly played by Mr. Butler and John Slattery, are undeveloped. Mr. Heald's character is, in the new version of the play, a book editor rather than a struggling playwright, but he still courts self-pity. Despite his supposed revelations, emotionally we learn little more about him than we knew in the first act.

At the same time, Mendy (Mr. Lane) does not appear after the first act. Mendy, the ultimate opera lover, the man who by his own admission is ''too much for most people,'' becomes a supporting player in a drama in which he has earned a starring role. In a difficult assignment, Mr. Heald artfully avoids overstatement. But it is Mr. Lane who deserves the highest praise for a brilliant performance as a man doomed to live an ordinary life while aspiring to the ecstasy of opera.

THE LISBON TRAVIATA, by Terrence McNally; directed by John Tillinger; sets by Philipp Jung; costumes by Jane Greenwood; lighting by Ken Billington; sound by Gary and Timmy Harris; production stage manager, Pamela Singer; fights staged by B. H. Barry. Presented by Manhattan Theater Club, Lynne Meadow, artistic director; Barry Grove, managing director. At City Center Stage I, 131 West 55th Street.

Stephen ... Anthony Heald

Mendy ... Nathan Lane

Mike ... Dan Butler

Paul ... John Slattery

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