Issue in setting up inputs and outputs in Logic Pro on iMac

ok, I would recommend to start first with only one soundcard. If that works, you can setup the other as well.

"26" audio channels means here you can have Simultaneous I/O up to 26 inputs / 26 outputs (at 44.1 or 48 kHz), with 8x8 analog and 16x16 digital (ADAT) + 2 SPDIF available.

8 + 16 + 2 = 26

But you might very likely only use the 8 analog in and outs.

Are you able to get one soundcard to work including accessing all 8 inputs and outputs in Logic?

If so, you can setup now the second one. Usually this should be done using control software provided by Presonus. I don't own presonus hardware, but it should work in a similar way to others.

Shane gave you great advice to use the Presonus Universal Control app. It's is an important first step. That app can install any firmware updates and drivers necessary to use them on your Mac. Check the documentation for those interfaces for details on setting them up on a Mac.

As for outputs, you can probably set Logic to use either one of those devices. Whichever one you have your monitors or headphones plugged into.

If for some reason you're only getting outputs from one of them and the inputs from the other one are not outputting, then you may have to daisy chain them together. Check their manual for instructions on that. A lot of times multi-channel interfaces like that have a connection for that purpose, such as an ADAT port on the back of each interface.

Hope that helps.

"...I did download the Universal Control app already. Do I use that for anything?"

Yes, you use that app to check that the firmware in both interfaces is up-to-date, and if not, you use it to update the firmware. Be sure to only have 1 interface at a time connected, when checking and updating firmware. The app has other uses, too, but since it's been years since I used my old Presonus interface, I don't remember what the other uses are.

I would definitely update the firmware, if necessary, before proceeding to my suggestion below.

In Logic Settings, cmd + , (comma), under Audio, Devices, select your Aggregate Device as the Input device and it should apply your ports/channels in numerical order and treat them as part of one big device. Also, once it recognizes your Input device, it will likely automatically select your Aggregate Device as Output, too.

Make sure you have downloaded and installed the Universal Control app. Refer to: https://support.presonus.com/hc/en-us/articles/4409668871949-Quantum-Installing-the-Quantum-drivers-on-macOS

Which sound cards are you talking about?