I’d appreciate any help with this simple question. I’m a holdover from the 80s who could make the ARP Odessey do anything I want but I’m have trouble keeping up with the digital age.
I bought a XF8 and pretty much only use the presets so far but I’m willing to learn. 
I’d like to be able to quickly (with the touch of one button) toggle in and out a second preset sound.  Specifically I want to layer in strings during certain parts of a piano based song.  I’m sure this is not very difficult but it’s less than obvious when I read the manual.

Although the following article was written for the Motif XS, it will apply in full to the Motif XF:

Using a MIXING setup for multiple sounds “live”

If you have questions or get stuck - post your question here in this thread.

There are also more involved ways to program the Motif XF to transition sounds that involve the XA CONTROL, if you find the above easy enough and want more you can try the more advanced tutorials:

Advanced Applications Tutorial: Assigning Controller Functions
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Voice Transitioning