Hello,
no, you don't have such functionality on the prophet. Env1, 2 and 3* are always triggered together (and thus, the keymodes only make sense in unison). As you said, using split can be a solution. But something I like to do is using velocity to inversely modulate the attack time, playing the chords softly with my left for slow attack, and the right hand notes fast for short attack. You could also test if using keyamount to modulate the attack could help you, but finding the right balance there might be tricky.
* unless env 3 is in repeat mode of course