How do I use different Soundfonts in different measures?

You can use different sounds (from different soundfontzs) in different instrument, not in different measures (unless you're using an instrument with multiple channels, like violin (acro, tremolo, pizzicato) or trumpet (open, mute), where you could use differently sounds (from differetn soundfonts) for different channels)

You mean, you have a piece in which you wish a single instrument to start playing a sound from one sounsfont, and then later one switch to using a different sound from a different soundfont? To do that, add an n instrument change from the Text palette, select the new instrument, then in View / Mixer, open up the channel strip for that instrument to show the sub-channel for the new instrument, and then change that to something from your other soundfont.