I think this does what you are asking:
#let grid-enum(e) = {
let sub-item-counter = counter("sub-item")
grid(
columns: (1fr, ) * 2,
row-gutter: .5em,
..e.children.map(c => {
sub-item-counter.step()
context sub-item-counter.display("(a) ")
c.body
})
)
}
// some list configuration here
#let sub-item-counter = counter("sub-item")
+ Here is my first item
+ Here is my second item
+ #[
#show enum: grid-enum
+ Sub A
+ Sub B
+ Sub C
]
+ Here is my third item
Note that used this way it does not support any further sub-enumerations.
There may be other issues with it, but for a simple case like this it seems ok.