EXS 24 Logic Sampler Users Group

Garth already explained the parameters wich you can change on a group level 
(actually, it's even a tad worse as you can in fact only adjust some offsets 
to what you have set on the EXS' GUI), so that's quite limited indeed.

In case you're doing such things more often, I can highly recommend getting 
another sampler. Inside Logic and for OSX, I'd say there's no better choice 
than Kontakt - which allows for doing all these things with ease. You 
wouldn't even have to create a new instrument out of two existing ones but 
smply load them into Kontakt on top of each other, then tweak the individual 
instruments to your likings. That way you can easily apply totally different 
envelopes, filters, FX and what not in a matter of seconds (or minutes, in 
case you're a deep tweaker).

Sure, that's quite some financial stretch, in addition Kontakt isn't acting 
as quick as the EXS, plus it's indeed less efficient, too - but after all, 
it's tweakers heaven, whereas the EXS is just a sort of workhorse sampler (a 
rather nice one at that, but not much more nontheless).