I have never done this before, but do they book one way flights? For half the points?

Most airlines, no. If you have specific airline in mind, tell us, or go check their website.

Hawaiian air sells one way ff tickets. I've paid between 16,000 and 25,000 miles depending upon the date. (A round trip ticket is 35,000 from Hawaii)

What I want to do is fly SW to Austin from San Diego and AA or DL from Dallas to SD. Doesn't sound like the one way with FF miles will work...

Several airlines (I know for sure USAirways) says they will book one-way - but at full RT mileage amount.

Yep, I tried booking a round trip with AA to Hawaii online to avoid paying the agent fees, but wanted to arrive Kauai and fly home from Maui. Could only book it as two separate trips - with a full 35,000 for each leg. So had to phone anyway and they were able to set it up for just the one 35,000.