One way is to use CartesianIndex
julia> a = rand(3,3)
3×3 Matrix{Float64}:
0.882002 0.04332 0.477523
0.829209 0.425031 0.0362383
0.303801 0.161715 0.422988
julia> a[[CartesianIndex(1,1), CartesianIndex(3,3)]]
2-element Vector{Float64}:
0.8820020685738753
0.4229883416619057