You should be able to alter the question to the following to get the final result:
SELECT distinct t1.RowID,
STUFF(
(SELECT ', ' + cast(t2.Field2Show as varchar(10))
FROM @SecondTable t2
INNER JOIN @FirstTable t -- join on @FirstTable based on LinkedField
on t.LinkedField = t2.LinkedField
WHERE t1.RowID = t.RowID -- associate the RowId from FirstTable to concatenate
ORDER BY t2.Field2Show
FOR XML PATH(''), TYPE).value('.','varchar(max)') ,1,2, '') AS ChildValues
FROM @FirstTable t1;