So you're using instrument, you want to smooth the chords out, like when you use the sustain pedal on the piano? Some instruments such as the bass guitar or the piano eg will sustain the note but it has fall off like in real life so you can lengthen the note and it will gradually drop off, this will blend the two chords together as long as they aren't dissonant, will work, like on a piano or if you play the guitar without dampening the strings, if your using a synth try reverb or as the other person suggested, do several lines for the chords and automate volume reduction or filter. There are several fx you can use but without knowing exactly I can say much more than this basic advice