absolutely. there is no reason you can't put humbuckers in series with each other, other than it might sound muddy. noiseless singles won't have that problem as they don't have much more inductance than regular singles.
if the pickups in question have any metal parts that are grounded make sure that they are grounded seperately. if the pickup only has a 2 conductor cable then you may have to modify the cable and add an extra ground.