Assuming you always have pair , the following should give you desired result:
SELECT a.event_id ,a.pair,a.versus,
a.score as score1, b.score as score2
FROM table_name a
INNER JOIN table_name b on
(a.event_id = b.event_id and b.pair =a.versus and b.versus=a.pair and a.pair<b.pair)
1. It makes sense to have a surrogate primary key column in the table.
2. The query would look much cleared if you had one more column which serves as pair identifier (ie. both 1-11 and 11-1 would share the same pair_identifier).
a.pair<b.pair added to avoid duplicates in resultset