My brother asked me
You can migrate directly from LeafGreen to SoulSilver. When you migrate the Pokemon, go to Fuschia City, where the Pal Park is located. Note: You can only do this after beating the Elite Four, and reaching Fuschia City.
LeafGreen and SS can't trade.
However, LeafGreen can migrate to Soul Silver. Just visit the Pal Park, which is in Kanto.