Alex - is it ok to use 2x long life 20" prefilters instead of just one?
You could, but I would not recommend it for the following reasons:
1. Only the first filter would actually do any chlorine removal for the life of the 1st filter
2. Although when the 1st filter 'runs out' the second could then start working, the 'spent' first filter would then tend to slow up the flow rate of water due to being old and having taken on sediment from the water
3. The flow rate of water through two units would be more restricted than through just 1 filter - this would affect RO production rates.