2 filters better than one?

I was thinking of hooking up a cartridge filter inline with my sand filter next year. I'm assuming I'd be right in hooking it up this way: Skimmer, pump, sand filter, cartridge filter, heater, return. My whole point is to catch the finer particles that the sand doesn't, while keeping the option of a multiport valve for vacuuming to waste or letting water out after rain, etc. Is this a possible option, and has anyone tried it? Would it affect my flow that much? I have a 1.5hp Hayward pump right now, with a 20,000 gal AG pool. Would I be able to get away with a smaller cartridge filter considering that I have the sand filter doing most of the work? Any recomendations on which kind to get? The DE trick never seemed to work for me, so why not try this right? Maybe I'm a little compulsive, but I want my water to look like its not even there:D Thanks!

I see no reason why the plan wouldn't work. As for the size of the cartridge filter, I would think it is determined by the flow rate and not the fact it's filtering pretty clean water already. You would take a flow resistance hit which is why bigger is always better for filters. I thought about this myself a few years ago before I tried the DE trick to get rid of fine dust and pollen.
Al

If you are going to do this, a better option is to split the flow afte the pump and plumb the filters in parallel. The slower water flow through both will trap even finer particles than having the cartridge after the sand filter.

Before doing this, why not try one of the cellulose fiber DE alternatives as a precoat on the sand filter. It works extremely well and would cost significantly less in labor and money.