Can't speak to HOU-IAH, but I've done DAL-DFW a couple of times.

Unfortunately, normally it runs twice as much dropping off at same airport, and I'm guessing it will be the same in Houston.

Of course one option is to drop off luggage and family at IAH, and then have only the driver go across town, which BTW can be done in Supershuttle (at least last time I did this a few years back).

Caveat, I've only done this online and never tried to call an agent, so you may have better luck there. Or not ....