Try to define an alias, as in
SELECT p1.id,
p1.name,
p1.beginDate,
p1.endDate
FROM SmartguideEntityBundle:Promotion1 p1
UNION
SELECT p2.id,
p2.name,
p2.beginDate,
p2.endDate
FROM SmartguideEntityBundle:Promotion2 p2
EDIT: If you take a look at this link, ti shows how to execute the query using RAW SQL (SQL query with UNION in Doctrine Symfony)