What Kontakt version do you have? I know for certain that version 4 couldn't do that, but version 5 can. I can't help you with Mojo Horns because I don't use that library.
Yes, you can enter two articulations for the same note, but the library must be able to handle that and you definitely need Kontakt 5. IIRC Mojo Horns has been available for several years, so it might not be 100% compatible with expression maps. Kontakt 5 has extra switches, but if the library was designed for an earlier version, they're not triggered. So, even if you use K5, it might still not work.
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