This is related to different posts in this thread, but I'll put them all in one post:
The value of having or using a ground instructor certificate when endorsing a student for a knowledge (written) test as a shield against a possible 44709 reexamination is zero. The student who repeatedly fails a knowledge test will run out of money long before it raises any flags at the FAA. If an applicant or series of applicants fail a practical (check ride) test, it could trigger a 44709 reexamination. There is a database that contains the CFI's performance, however, that data is only rarely examined and even less often acted upon. Typically when a CFI receives a 44709 reexamination for something related to their student (as opposed to an event in which they were in the aircraft), it is when the student is involved in a problem (airspace bust, accident, etc.). In one rare instance, I saw a 44709 done on a CFI for paperwork. That was a CFI who kept signing 90 day solo endorsements on the student's certificate instead of their logbook. A couple DPEs tried to explain to him that he was doing it incorrectly, but due to the CFI's "unique" personality, he was convinced he knew more than the DPEs. The DPEs mentioned it to the FAA at a meeting and the result was a 44709 of the CFI to evaluate his understanding of logbook entries and pilot certificate endorsements.
To obtain a ground instructor certificate the applicant needs to present their knowledge test for that particular certificate along with a Fundamentals of Instruction (FOI) knowledge test or one of the items listed in 14 CFR 61.213(b). The most common way to do that is to go to the FSDO. (Be sure to check ahead of time if they require appointments.) However, if there is a hardship involved in doing that, it may be possible to get a ground instructor certificate from an examiner. There is a provision in FAA Order 8900.1, Volume 13, Chapter 3, Section 1 that allows for "Ground Instructor Examiners" (GIE). If done correctly, it requires the person to be designated in writing. However, it is often done by a phone call between the FSDO and the examiner. Of course, you want to sort all that out ahead of time instead of surprising them with the request, particularly since only one in five FAA inspectors even knows what a GIE is.
As to the OP's question about would it make the test harder if he tried to do both at the same time, the answer is probably yes. If the test is administered by a DPE, it is likely the DPE wouldn't have a clue what the applicant was talking about. However, if it were given by an FAA inspector, the inspector would have the authority to issue the ground instructor certificate and hopefully would understand the process and the requirements. But, to be on the safe side, I would probably do that in a separate visit to the FSDO. In fact, if the FOI test score is passing but not good, getting the ground instructor certificate ahead of time would be a good way to keep from having to show the low test score on the FOI. On the other hand, if the FOI test score is high, make a copy of the test and get the ground instructor certificate ahead of time. Then show the ground instructor certificate and casualy present the copy of the FOI with a statement such as "I know this isn't needed because of the ground instructor certificate, but here is a copy just in case you wanted to see it".
As to the value of a ground instructor certificate, that is an individual question. Many certificates are obtained for no reason other than bragging rights. It does make the instructor eligible for a gold seal, which made sense back in the day that tests were given for free. However, with today's test fees (which are about to get even more expensive), the decision becomes more one of personal economics.
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