Each Develop preset (I presume those are the presets you're referring to) contains a list of values for the various controls (whichever controls were included in the preset).
But they're absolute values, not relative ("set value to X", not "increase value by X"). So no, there's no simple way to combine them.
As suggested, you could apply one preset, create a TIFF/PSD of it (not a JPEG) and then apply another preset to that. But you'd be losing the advantages of parametric editing (what happens when you want to go back and tweak settings on the original RAW?). I do NOT recommend this path.
There's no interface to show you exactly what's in a Develop preset, but if you can work out how much the settings are changed, you could try manually doing those changes to an image that's already had another preset applied, and eventually work out the resulting set of appropriate combined values (then save them as a preset for future use).
You can either find out what's in the preset by examining which sliders move where when applying/undoing the preset, or if you're a bit more "techy" you can go and look at the values inside the preset's .lrtemplate file (it's XML and vaguely human-readable).
I've never been keen on the supplied Develop presets, especially when teaching Lightroom to students. They encourage a "click-n-pray" experimental technique to edit images, rather than encouraging users to understand what's actually going on. They can provide useful examples of what can be done, but it's worthwhile then going through and working out how to do it yourself!
