Air conditioning condensation drain lines

My girlfriend just bought a house (built in 92), and the air conditioning condensation drain lines both drain when the unit is running. I thought the auxillary drain line only took over when the primary line that runs into the sink drain became clogged. I went up into the attic and noticed both PVC lines come out of the drain pan at the same level, suggesting that they would both drain at the same time. I guess this isn’t that big of a deal, but the home inspector she hired said the aux. line shouldn’t be draining. He never actually went up into the attic to look at the unit, however. Do you have any idea if this is a typical installation? If so. does she just live with the aux. line dripping from the eave all the time (it has rusted the flashing on the second floor siding-roof tie in.) I would appreciate any comments you might have–thanks!