Si solo es sumar sl_pri + (selec....)
Código SQL
[-]select sl_pol,sl_pri,
(select sum(en_pri)
from endosos
where en_pol=sl_pol and
en_cia=sl_cia and
en_vig2=sl_vig2) as totend ,
sl_pri+coalesce( (select sum(en_pri)
from endosos
where en_pol=sl_pol and
en_cia=sl_cia and
en_vig2=sl_vig2),0) as total
from polizas p1
where (1=1) and sl_num like 1305