OK, to answer my own question (typo's possible, works here ;-)):
CREATE TYPE abonnementartikelheader AS (col1 int4, col2 int4);
CREATE OR REPLACE FUNCTION getabonnementartikelheader(int4)
RETURNS SETOF abonnementartikelheader AS
'
select
(SELECT COUNT(klantnummer)::int4 FROM abo_klt WHERE abonnement = $1),
(SELECT SUM(aantal_abonnementen)::int4 FROM abo_klt WHERE abonnement = $1);
'
LANGUAGE 'sql' VOLATILE;
Groeten,
Joost