Group element by another element?

The following SQL

with t(a, b) as (select 1, 2 union all select 1, 3)
select  t.a as 'X/A/@Id',
        t.b as 'X/A/b'
from    t
for xml path(''), root('ROOT')

returns

<ROOT>
  <X>
    <A Id="1">
      <b>2</b>
    </A>
  </X>
  <X>
    <A Id="1">
      <b>3</b>
    </A>
  </X>
</ROOT>

Is it a way to group the two <b> into <a> since the Id(a) are the same? (Is it a way to do it without using subquery if possible?) Expected,

<ROOT>
  <X>
    <A Id="1">
      <b>2</b>
      <b>3</b>
    </A>
  </X>
</ROOT>