1. How to set up Audio in Montage to send audio data to Logic?

You need to download and install the “Yamaha Steinberg USB Driver v2.0.3 for Mac”
Connect your MONTAGE “To Host” port to your Mac (using the appropriate adapter).
This single driver will handle both MIDI and Audio communication between the MONTAGE and your computer.
For MIDI communication to take place [UTILITY] > “Settings” > “MIDI I/O” = USB (green)
Your Mac will see the MONTAGE as an audio interface 32-in/6-out (the first pair of USB Inputs are the MONTAGE Main USB L/R; These are followed by 30 Assignable buses (USB1-USB30) — these can be setup as odd/even stereo pairs or as individual buses, as you may require).
Link: __ Official MONTAGE Download page

2. Which output is needed? Is one of the USB-Stereo-Outputs posible to send the sounds into the Sampler?

The first pair of MONTAGE USB Outs (seen by the computer as the first pair of USB Ins) — Main USB L and Main USB R (your program, LOGIC, May only see them by number, 1 thru 32 instead of “Main L/R” + “USB1-30”.
Just use the first pair — they are the main stereo outputs. All Parts default to being routed to this pair of Outputs, by default.

Each MONTAGE Part can be routed to Outputs. From the HOME screen:
Press [EDIT]
Select the Part you wish to route, press [PART SELECT 1]
Touch “Part Settings” > “General”
The “Part Output” is shown on this screen — if you tap the current setting “Main L&R” a pop-up menu will show you your choices.
Each of the sixteen possible Parts has the ability to be routed to any of the available Outputs

If you select a Drum Kit Part, this “Part Output” can be set to “Drum”.
This allows you to route each Drum Key to an assignable Output on a Key-by-Key basis (C0~C6) — great for when you want record individual Drums to their own discreet Audio Track.