I spoke to a Sofer today who told me over the following shailo.
When he writes Shatnez Gatz he always writes the tagim first then the ois. This he was makabel from one of his Rebbes in STaM (who I happen to know to be an unbelievable Sofer.)
He said he wrote the tagim first however they became connected at the bottom forming almost like a Shin. He then wrote the ois further and realized afterwards what had happened. His question is since when he wrote the ois (and connected it to the tagim) the tagim were "pasul" (looked more like a very small shin more than tagim.) Is there a problem here that the ois was nictav b'pisul? Would there be a problem now of chok tochos in regards to the ois to just erase the tagim? Seemingly it doesn't change the tzurah at all of the ois and just looks like 2 oisios that are touching. Therefore chok tochos I don't think would be a problem. However the nictav b'pisul part is really the question. Perhaps he should go back now and erase everything (as opposed to just erasing the tagim.)