net1 = NetGraph[
 {
   "2x" -> ElementwiseLayer[(2*#) &],
   "sum" -> ThreadingLayer[(#1 + #2) &]
   },
  {
   NetPort["a"] -> "2x" -> "sum",
   NetPort["b"] -> "sum"
   }
  ]

net1[<|"a" -> {1, 2, 3}, "b" -> {10, 20, 30}|>]

net1[<|"b" -> {10, 20, 30},"a" -> {1, 2, 3}|>]

net2 = NetGraph[
  {
   "A" -> PartLayer[1],
   "B" -> PartLayer[2],
   "net1" -> net1
   },
  {
   NetPort["Input"] -> "A",
   NetPort["Input"] -> "B",
   {"A","B"} -> "net1"
   }
  ]
net2[{{1, 2, 3}, {10, 20, 30}}]

{<|"b" -> "b", "a" -> "a"|>} -> "net1"

net2 = NetGraph[
  {
   "a" -> PartLayer[1],
   "b" -> PartLayer[2],
   "net1" -> net1
   },
  {
   NetPort["Input"] -> "a",
   NetPort["Input"] -> "b",
   "b" -> NetPort["net1", "b"],
   "a" -> NetPort["net1", "a"]
   }
  ]