[UPDATE: January 7, 2019]
Transunion finally updated. New FICO 8 Score: TU 689.
It looks like all the scores are stable now. I'll wait until February 11 to generate a new myFICO monthly report (I will still get FICO score changes when they happen, just without the detail of a full report), to allow time for Transunion to update after both of my card statement closing dates. Card 2 closes the day after Card 1.
These snapshots show the result of a loan closing, and new revolving accounts showing up at each bureau.
It's basically around a 30 point drop on this scorecard for closing a loan. [EDIT Clarification: It's a FINAL ~30 point loss after a HUGE DROP for 1 loan closing without any revolving accounts on the report (-81 points at Experian for instance, I'm unsure how much a single loan drop is for the other 2 bureaus because I don't get that level of detail with myFICO alerts) followed by a HUGE GAIN for adding a new revolving account.] Huge gain for adding 1 new credit card, and the second card added right after doesn't seem to have any effect at all.
The next FICO score changes will be from 2 on-time payments reporting (to $0 balance of course), a 6.75% utilization on Card 1, and 7 to 8% utilization on Card 2. I'm done using Card 1 for this billing cycle - it has $135 on it out of $2000. I just started using Card 2 for gas and groceries, and will continue to use it up to a maximum of $520 out of $6500, which is 8% utilization. These credit cards are merely a vehicle for proving that I always pay my bills on time and that I am extremely fiscally responsible.
I use the Citi Costco Card at the pump as my main method to purchase gas.
You do not need to use the membership card- the Citi card works as both membership & payment.
Thank you, Pikaboo - you are right, I only had to use the one card. I only ended up looking like an idiot at the Costco gas pumps for 10 seconds, because apparently I got the one gas lane reserved for peasants and it didn't have Visa payWave. So I was looking for it on the pump while the guy behind me was probably saying, "Oh great, another new member at the gas pumps for the first time."
Just adding this for anyone else browsing: Another cool thing is when I got to the register, I just handed the guy the Citi card instead of membership card, and after he scanned everything he just asked me if it was ok to wave it over the terminal. I said "Yes", got my card and receipt and on my way. Pretty cool.