Interpol vs. Schengen Information System (SIS)

Interpol is a mechanism for aiding police cooperation via the diffusion of information. They maintain databases of wanted persons, stolen or lost IDs etc. In europe SIS does much the same thing, except it is limited to 15 european countries, all of which are members of Interpol. This redundancy could be eliminated by enlarging the SIS database to include that of interpol. This would mean that searching the schengen database would also turn up interpol notices. Otherwise, it would be necessary to search both the schengen and interpol databases at customs control?

Does anyone know how this is dealt with in practice?