Mark Twain visiting Nikolai Tesla’s laboratory.
Which reminded me of awesome people hanging out together. Scroll down far enough and you’ll find (a) Andy Warhol met everybody and (b) moments like Theodore Roosevelt hill walking in Yosemite with John Muir and the surprising sight of Amelia Earhart admiring Harpo Marx’s costume on the set of Horse Feathers.
And perhaps the ultimate scientific example, the Solway Conference.
Generally one historical celebrity (or one group generally thought of going together, like a political movement or an artistic or musical group) is enough and provides plenty of characterisation, so a further “guest” star might just complicate matters unnecessarily, but a meeting could be the cause of the adventure, or a gag made in passing. I’ve used the Shelleys and Byron at the Villa Diodati before (as have Big Finish and Doctor Who Adventures) but which other intersections of history could attract the attentions of time travellers and alien invaders?