The Motif XF is a synthesizer in addition to everything, so you can customize it to a great degree. And you can approach the problem from several different directions.

A Motif XF VOICE is made up of eight Waveforms. Multiple Voices can be placed together in Parts of a PERFORMANCE.
If your Piano and Strings are created in a Voice, by using four Elements for Piano and four for Strings, you can accomplish your goal by addressing just the tuning of the String Elements. Coarse Tuning +16 = 1 octave.
If you construct your Piano and String program by combining two Voices into a PERFORMANCE, one Part for Piano, one Part for Strings... then you can accomplish your goal by addressing just the tuning of the String PART. Note Shift +12 = 1 octave.

ALTERNATE METHOD
By placing the Piano and String programs in a Song or Pattern Mixing Setup, you can seamlessly switch between one Piano/String layer and another using two adjacent [PART SELECT] buttons to recall your setups. This method can accommodate what you want to accomplish without much trouble and has the added benefit of not having to cutoff the sound of held notes when move from one Piano/String setup to the other.

Place your Piano and String sounds in a blank SONG MIXING.
Use the ability to assign MIDI Receive Channels, so with Part Select button 1 you are recalling the Piano with Normal Strings, with Part Select button 2 you are recalling the Piano with the Strings an octave higher.

Because in a MIXING each button selects a Transmit channel, you can stack the sounds you want to play when [1] is selected on MIDI Receive Channel 1. And similarly, you can stack a Piano and an octave shifted String on Receive Channel 2. This way when you are performing you can switch using just those two buttons [1] and [2].

We are not sure if you are using a Voice and don’t want to or for some reason can’t use a solution that involves Performance Parts or Song/Pattern Mixing Parts. Let us know.

Thank you for your quick reply.
And sorry for being unclear:

I would like to do it in Performance Mode. I know that I could solve my problem by easily switching between to performances. One with the strings set "low" and a second having the strings set one octave higher.

My question was directed to the controlling options of the XF: Modwheel, AF 1&2.
I have to do the octave switch in a second, very quickly.

My goal is to set the "octave switch function" to one of the controllers!

And then disable it for the piano voice...