Use both decals if you are doing a pre-war or early-war helmet. After experience in Poland and France, an order in 1940 called for the removal and omission of future tri-color shields (red-white-black striped shield for Heer & Luftwaffe, shield with swastika for the SS). The bright colors were just too visible. Of course, as with any military, this was apparently adhered to only when necessary... there is plenty of photographic evidence showing double-decal helmets all the way to the end of the war.
An order dated 1943 called for the removal and omission of ANY decals on helmets. This would mean that ideally-speaking, the 1942 run of M42 shells would have had the National Eagle (or "SS" runes) decal applied in the factories, but after 1943 no decals should be seen on new helmets. In addition, many M35 & M40 shells were reconditioned and reissued all the way to the end, so these rules would apply to them too. To add further confusion, decals were available for private purchase, so it is conceivable that a sneaky Soldat might have applied his own double-decal job to his spanky-new M42 for whatever reason! The best late-war look is probably no decals at all.