President Reagan will formally endorse Vice President George Bush, who has all but clinched the Republican nomination, in a joint appearance in the next couple of weeks, White House spokesman Marlin Fitzwater said Thursday. Fitzwater said the president "will endorse the vice president, but we don't have a specific date . . . probably in the next couple of weeks." He said there will be a "special ceremony" for the endorsement, which will also kick off Reagan's active participation in the campaign.