Your TOML is rather convoluted, a more normal way to write it would be:

  [a]
  b = [{x = 1}]
  y = 2

Or alternatively:

  a.b = [{x = 1}]
  a.y = 2

Some parsers allow newlines in TOML inline tables, so you can do:

  a = {
    b = [{x = 1}],
    y = 2,
  }

That's supposed to be in the next TOML standard, but that seems indefinitely stalled as the (only) maintainer has seemingly lost interest and there hasn't been any movement for a long time.