A two stage system is two housings plumbed in series, and it is the arrangement that gets the most out of the least equipment. The reason is division of labour: the first stage takes the load that would otherwise destroy the second, and the second does the work the first cannot. Which two stages you choose depends on what the water is carrying.
Sediment then carbon, the standard pairing
The most common arrangement puts a depth sediment cartridge first and a carbon element second. Sediment first is not a preference: particles blind the carbon's surface and waste the media inside it, so reversing the order throws away most of the carbon's value. With the sediment stage doing its job, the carbon element sees only clean water and spends its whole capacity on chlorine, taste and organics, which is what it is for and what it is expensive for.
Coarse then fine, for gritty supplies
On well water carrying real sediment, both stages are sediment: a coarse depth cartridge first to take the bulk, and a finer one behind to polish. That combination lasts many times longer than a single fine cartridge, because most of the mass in gritty water is in the large particles and catching those cheaply protects the expensive element. Households that fit one fine filter and change it monthly usually solve the problem entirely by adding a coarse stage in front.
What two stages cannot do
Neither arrangement softens water, removes dissolved iron or manganese in useful quantity, addresses nitrate or dissolved salts, or disinfects. Two stages of filtration is still filtration. If the water needs hardness removed, that is ion exchange; if it needs iron, that is an oxidizing filter; if it needs organisms dealt with, that is ultraviolet or chlorination. A two stage system is often the foundation those sit on rather than an alternative to them.
Fitting it so it stays serviceable
Isolation valves either side of the pair and a bypass across both, so a cartridge change is ten minutes rather than an afternoon without water. Unions at each connection so a housing can come out without cutting pipe. Clearance below each sump equal to a cartridge length. Pressure gauges before and after, which turn changing the first stage from a guess into a number. Our whole house installation guide covers sizing both housings for the household's peak flow.
Questions people ask about 2 stage water filter system
Which stage goes first?
Sediment always before carbon. Reversing them lets particles blind the carbon surface and wastes the media inside.
When should both stages be sediment?
On gritty well water: a coarse cartridge first then a finer one, which makes the fine element last many times longer.
Does a two stage system soften water?
No. Two stages of filtration is still filtration. Hardness needs ion exchange, which is a different kind of equipment.
What should I fit alongside it?
Isolation valves either side, a bypass across the pair, unions at each connection and gauges before and after.