There are several remedies:
- EQ on side with more low end and the other with more mids (cuting the oposit side helps)
- Put the keyboard on one side and put a delay of the signal to the other side (20-60ms)
- Use a Chorus/Flanger/Vibrato/Tremolo/Reverb (you also could use it the way like the delay above)
- Use a "Make It Stereo" plug like the one from Waves
- Compress one side only or use something like a Transient Designer with oposit settings for L/R
- Use 2 different sound modules/keyboard/patches/sounds for left and right
Use all the above ideas in different combinations.