The way you want to phrase the query is to have no subqueries at all in the select
clause:
select sum(case when username like 'a%' then score end) as scoresForANames,
sum(case when username like 'b%' then score end) as scoresForBNames
from (select username, score
from gameTable
where gameId = 111
) derivedTable;
Of course, in this case, you don't need the derived table at all:
select sum(case when username like 'a%' then score end) as scoresForANames,
sum(case when username like 'b%' then score end) as scoresForBNames
from gameTable gt
where gameId = 111;