How do the tastes of the first and second pulls compare? If the water is too hot or too cold for the second, you'll taste it as bitterness or sourness respectively.

Do you clean the basket before loading the second shot? The description of your technique has you wiping the basket with a rag before the first shot, but not before the second. A wet basket -- especially one with a few fines around the sides -- can cause channeling. Quite often (usually?), premature blonding is caused by channeling.

How long does the second shot take relative to the first in order to extract the same volume? Everything else being equal, a faster pour could be another indication of channeling.

What do the first and second pucks look like after their respective pours? How do they come out of the pf when you knock them? That is, does one usually hold together while the other breaks up? Does one seem significantly wetter than the other?

If you can at all afford it, you might want to invest in a naked pf, to learn to diagnose and improve your loading and tamping techniques. It's not only worthwhile as a training tool, the naked pf makes for a visually interesting pour.

It's interesting to note that most everyone finds the first shot of the day from a given machine to be the worst. They (the machines, not everyone) seem to need a little warm up before getting their groove on.

Good luck,
BDL

A video of both extractions would be helpful.

Early blonding on the second shot points to puck preparation inconsistencies leading to channeling. To confirm, get a 0.1 gram accuracy scale and use the foolproof WDT + Stockfleths Move for Dummies. It's a little OCD but instructive to get two identical baskets and prep them outside of the portafilter (remove the retention clip so they drop in easily). Then you can confirm or refute with certainty if the issue is tied to the first shot/second shot preparation by preparing both at the same time, then having a helper give you one or the other to brew at random.