query with union

You can use Common Table Expression

WITH myUnion as
(SELECT
  custom_qui.QuiID,
  custom_sb_quoi.numqui,
  custom_sb_quoi.CPAct1 AS CP,
  custom_SB_quoi.Latitude_act AS Latitude,
  custom_SB_quoi.Longitude_act AS Longitude
FROM custom_qui,
     custom_sb_quoi
WHERE custom_qui.quiID = custom_sb_quoi.numqui
AND CPAct1 IS NOT NULL

UNION ALL

SELECT
  custom_qui.QuiID,
  custom_sb_quoi.numqui,
  custom_qui.CP AS CP,
  custom_qui.Latitude AS Latitude,
  custom_qui.Longitude AS Longitude
FROM custom_qui,
     custom_sb_quoi
WHERE custom_qui.quiID = custom_sb_quoi.numqui
AND CPAct1 IS NULL)
select *  from myUnion WHERE ##WHERE## ORDER BY ##ORDERBY##

P.S. if the query above works well - you can simplify it- i.e.remove your conditions (CPAct1 IS NOT NULL and CPAct1 IS NULL - they combined give you the same result set as if you don't use them) and merge everything into 1 query

This should give the same result as the query with union above:

SELECT
      custom_qui.QuiID,
      custom_sb_quoi.numqui,
      custom_sb_quoi.CPAct1 AS CP,
      custom_SB_quoi.Latitude_act AS Latitude,
      custom_SB_quoi.Longitude_act AS Longitude
    FROM custom_qui INNER JOIN custom_sb_quoi on custom_qui.quiID = custom_sb_quoi.numqui
    WHERE ##WHERE## ORDER BY ##ORDERBY##