@Fionja , per your example , at sometime during the year both account A and B reached US$200,000. So for purposes of ONLY threshold ( to need to file ) determination each account must be reported as US$200,000. This reporting has NO TAX implications. If these are bank accounts i.e. liquid assets then you also have to report FBAR -- generally FATCA reporting does not exclude need for FBAR reporting.
Here is a summary comparison of FBAR and FATCA --> Comparison of Form 8938 and FBAR requirements | Internal Revenue Service
Is there more I can do for you ?
