On 2008-07-07, Jim Rainfordson wrote:

Now I want to hook them up so that water flows separately through
each, then joins back together again before exiting the kitchen
faucet.

Sounds fine, it would be best to ensure the parallel paths are
identical, same length of fittings, etc.

The unit is housed with 3/8" FPT (plastic) holding a 3/8MPT to 1/4
compression male (plastic). Do I have to step it down to 1/4 or can
I use the 3/8" FPT? I'm wondering if putting too much pressure on
the unit (by not bottlenecking it to 1/4"C) will cause it to
function improperly.

I would be inclined to use the unit as supplied with the 1/4" fitting.
Increasing the pressure on the filter would increase the flow rate,
and it is possible this could decrease the filtering efficiency. You
could check with the manufacturer to see what they say.

What about exiting the units? If 1/4 C is the intake then that's the
bottleneck, so at that point it shouldn't matter if I use 1/4"C or
3/8" FPT, right?

No, that's not really true. The way fluid flow works with
pipes/fittings in series with a constant pressure source is that each
fitting will have a certain flow rate to pressure drop relationship.
The overall flow rate will be just enough so that the sum of the
pressure drops across all the fitting/pipes between the source and the
faucet will just equal the pressure of the source. So adding
additional lengths of small diameter tubing will cause additional
pressure drop and decrease your overall flow rate.

Yours, Wayne