I got a reply back:
Hi Simon,
I don’t see how it can be reducing the life of the second unless you have bacteria growing in the first tank over wise it wouldn’t make any difference - resin has a set capacity once its met its depleted the only way to shorten its life is to introduce more impurities into the water as long as that doesn’t happen the life of the second resin vessel will not be effected.
Kind Regards
June