My recommendation is to install an EFIE the one in front of the O2 sensor first.
MOST vehicles don’t use the rear oxygen sensor as input for fuel mixture (it’s only for checking the operation of the catalytic converter).
You can confirm this with your vehicle manufacturer, or just wait and see if the front only EFIE is effective.
But if yours is a vehicle that does use the second one as part of the fuel mixture calculation, you will have to add a second EFIE.
For easy instruction (and to sell more EFIEs) a lot of EFIE manufacturers just recommend installing on both.
It doesn’t hurt to have an EFIE on the rear oxygen sensor even if it isn’t used to calculate to calculate fuel mixture.