To be honest, this kind of event is often specific to certain limited series or special story arcs. It could vary depending on different comic runs and creative teams. But I can't pinpoint exactly which one off the top of my head.
As far as I know, in the traditional Marvel comic storyline, Tony Stark doesn't transform into Dr Doom. They have their own separate identities and story arcs.
In many Marvel crossover events, comics like Secret Wars or Civil War, you can find both Tony Stark and Dr Doom. These stories bring together various heroes and villains, creating complex and exciting plots.