Without using a laptop (since I don't have one and will be playing live) is it possible to make the top half of a midi keyboard play sax while the bottom half plays a different synth sound? My low budget setup is a midi keyboard connected to a Roland JV 1080.
I can't answer whether or not your keyboard can do this, however a lot of midi keyboards have a 'split key' mode which allows precisely this.
The best advice is to google the specific make and model of your keyboard followed by the word 'manual'. Most gear has a digital version of it's manual available online, and this should tell you if it has 'split key' and how to activate it. Most often this means that keys above a certain part of your keyboard will pass to one midi channel and below will pass to another.