Former President Donald J. Trump and his personal aide, Walt Nauta, were ordered by a federal magistrate judge on Tuesday to not discuss their criminal case, even though the two work closely and see each other practically every day.

Magistrate Judge Jonathan Goodman, who oversaw the hearing, said that any discussions related to the case must go through their lawyers. Mr. Trump and Mr. Nauta have been charged with conspiring to obstruct a federal investigation into Mr. Trump’s handling of dozens of classified documents after he left office.

Mr. Nauta did not enter a plea. A lawyer for Mr. Nauta, who is charged with lying to investigators and scheming with Mr. Trump to conceal boxes containing classified documents from the F.B.I. and federal prosecutors, asked for a two-week extension because he needed a local lawyer to sponsor him.

The restrictions — which do not apply to other topics of conversation — are common for co-defendants in a criminal matter, but they could be particularly challenging to uphold given that Mr. Nauta’s job is to follow the former president through his days, attending to various needs. To underscore Mr. Nauta’s proximity to Mr. Trump, Mr. Nauta was riding with him from Mr. Trump’s club, Doral, to the courthouse for Tuesday’s hearing.

The two men talk frequently and have for most of the last two years as Mr. Nauta first served as a valet in the White House and now serves as an aide to Mr. Trump in his post-presidential life. The former president tends to treat his close personal aides as sounding boards for all manner of topics.

Mr. Trump is hardly known for his restraint under typical circumstances, but especially when told to do something by a person in a position of authority. And an edict not to discuss a case that has consumed Mr. Trump’s thinking for weeks poses even more of a challenge.

The same restriction on the defendants’ communications was also applied to witnesses in the case, a list of which the government is expected to draw up. That poses a similar challenge to the situation with Mr. Nauta: A number of Mr. Trump’s advisers, current and former Mar-a-Lago staff members, and even some of his lawyers have been interviewed in the case.