For years in fashion, it was trendy for one designer to helm two brands: their own, usually a relatively small one, and a big heritage house at which they acted as, essentially, talent for hire. John Galliano did it, so did Alexander McQueen, Alexander Wang and Karl Lagerfeld (well, he actually ran three brands).

Then, as tends to happen, industry wisdom decided this was not really a good idea. It was too draining, too complicated and a designer should be fully committed to one brand. Then after awhile, everyone got bored with that theory, decided they were the exception to the rule, and embraced multitasking again: Gabriela Hearst did Chloé and her own line (though she is leaving Chloé after this season); Jonathan Anderson does JW Anderson and Loewe.

But exactly why it’s so hard for one designer to serve two masters was rarely more obvious as it was on Tuesday at Peter Do’s fantastic first show in Paris — which took place about two weeks after his ho-hum first show as creative director of Helmut Lang, in New York.

Mr. Do had jumped cities with the eponymous brand he founded in 2018 (it is based in New York, and had been showing there), in part to put some space between it and Lang. But while there might have been an ocean in between, conceptually and aesthetically, they were intertwined.

In many ways, the Peter Do show was more in the spirit, if not the exact shape, of Helmut Lang than the Helmut Lang show. It took the Lang legacy, made it Do’s own, and then moved it forward.

It had the slouchy, slick pants hanging just so off the hip bones that are a Do signature and that are firmly in the Lang tradition, paired with halter tops lined in leather, like notional tuxedos. The pleated kilts that could be belted over skinny trousers, and the sweeping white shirts (which were actually part of a collaboration with Banana Republic). The color blocking, here as vertical and horizontal bands of black, white and red tracing raceways across the waist and down one leg.

But it also had its own lewdly tempting edge: banana pants slit open at the thigh to expose transparent chiffon sweats layered beneath; a T-shirt slashed from collarbone to waist to open like a mouth, exposing another transparent layer inside; a long, skinny crimson tube of a sheer skirt with a black stretch-knit waistband worn with a white shirt tucked in so the tails covered, just barely, the nether regions.

Watching it, there was no question why Lang’s owner, Fast Retailing, had wanted to hire Mr. Do. But it was also impossible not to think that perhaps there are only so many really good ideas per season and Mr. Do — understandably — saved the best for his own line.

Maybe he will grow into balancing the two, and the differences will become more defined. But maybe, rather than endlessly trying to restore storied old brands to their former glory, it would be better if everyone (backers, retailers, consumers) were willing to invest in building new ones.

Because, really, one great Peter Do show should be enough.