Our two man set up is: tank - pump - DI - DI - Y splitter - reels.
You'll need a 100psi pump for two reels off one pump. We've been doing it this way for 15+ years without issue, yes you get a bit more pressure off one reel whilst the other is unplugged but it's not a problem, pumps will need replacing more often but we're still talking years.
A double DI set up is to save resin, no other reason, a carbon filter won't help.
As soon as you get 001 out of bottle 2 move it to position 1 - it then feeds your refreshed bottle 2 with semi purified water. By the time bottle 2 is giving you 001 you know that the resin in bottle 1 is totally spent.
If you have only one di, whether you have a prefilter or not, you'll be chucking resin away that's still partially purifying the water to 8ppm when your pump shuts off.