For Python 3.12, Godbolt gave almost identical bytecode for both (albeit in different order.) I'm guessing wildly but might this be because `BINARY_OP(+=)` stores the result (because it's `INPLACE`) and then you also do `STORE_FAST(x)` which gives you two stores for the same value compared with one store in the single-line version?

Single-line dual assignment:

    2         2 LOAD_FAST                0 (a2)
              4 LOAD_FAST                1 (c)
              6 BINARY_OP                0 (+)
             10 LOAD_FAST                1 (c)
             12 LOAD_CONST               1 (2)
             14 BINARY_OP                0 (+)
             18 STORE_FAST               1 (c)
             20 STORE_FAST               0 (a2)

vs the two-line version:

    2         2 LOAD_FAST                0 (a2)
              4 LOAD_FAST                1 (c)
              6 BINARY_OP               13 (+=)
             10 STORE_FAST               0 (a2)

    7        12 LOAD_FAST                1 (c)
             14 LOAD_CONST               1 (2)
             16 BINARY_OP               13 (+=)
             20 STORE_FAST               1 (c)