The news is being reported in detail on a number of business-focussed webpages, so rather than attempt to repeat it, here are a number of relevant links.
Financial Times: National Expess in talks to buy Stagecoach
The Guardian: National Express in talks to buy rival Stagecoach
London Evening Standard Stagecoach and National Express in merger talks
Daily Business: National Express in talks to acquire Stagecoach
All reports confirm that National Express would be the major partner in the combined group and most suggest that the motivation for the move is the fall out from the pandemic, which has seen both companies' share price halved and huge losses in passenger numbers over the last eighteen months.
Under takeover rules, National Express has until October 19th to confirm its bid.