"Reality Storm: When Worlds Collide" crossed over Hero Games' Champions and Guardians Of Order's Silver Age Sentinels, co-produced by both companies. The adventure itself was dual-statted, but the book also included official conversion notes between the Tri-Stat d10 system and 5th edition Hero System.
The Spelljammer novels of The Cloakmaster Cycle demonstrated such a campaign, with most of the first book taking place on Krynn, only gradually introducing the concept of spelljamming to the hero. Later he visited two of less known locations on Toril. At least Forgotten Realms actively supported the meeting with other worlds by dropping mentions in its own sources.
West End Games' Paranoia had a trilogy of crossovers under the banner "Vulture Warriors of Dimension X". The first two adventures crossed over with Cyberpunk 2013 and Twilight2000, while the third adventure used time- and dimension-travel as a campaign hook for open-ended crossovers with other franchises.
Warhammer 40K and Warhammer Fantasy were originally much the same (the former being the latter Recycled In Space), but drifted apart with further editions with only Chaos and the Warp linking them together. Nevertheless, they are occasionally hinted to be in the same universe, with futuristic items and artifacts finding their way into the Chaos Wastes, and KaldorDraigo making an appearance as well.