We have to assume that Speck came first, with “Speck’s Idea” published in 1979.
“Overhead in a Balloon” was published five years later (both stories in the pages of “The New Yorker”, where the majority of Mavis Gallant’s stories appeared before they were bound into collections).
So we have to assume that Mavis Gallant wasn’t convinced of Speck’s take on his gallery assistant, that she wanted to get to know Walter herself.
Speck, readers learn in this collection’s first story, spent half of his time envisioning the final act of firing his assistant Walter. He hasn’t completed the act, however.
In turn, when readers spend time in Walter’s mind, we learn that Walter spends half of his time abandoning his employer (or in seeing him held hostage or otherwise suffering, no longer a threat to Walter in any case).
Speck is awake and planning; Walter is asleep and dreaming.
Neither man admires the other, both men fantasize about their relationship ending: the relationship carries on.
And what is the root of their discontent?
For one thing, they do not view the gallery business from the same perspective.
Consider this scene:
When Aymeric brings his portfolio to Speck, in hopes that Speck will consider hosting a show of Aymeric’s paintings, Speck is immediately dismissive. Meanwhile Walter is determinedly gesturing and nudging, ultimately dismissive of Speck’s dismissal. (Later when Walter and Aymeric meet to discuss how Walter might help Aymeric with his portfolio, nothing comes of Walter’s good intentions.)
Speck is in charge; Walter is the assistant. Neither has as much influence as he “deserves”, not in the art world, not even with one another.