Can't answer first question but theoretically the second could be answered (have not tried it).
1. Download first sketch.
2. SSH to Edison
3. Move sketch from /sketch to a nice safe place - call it 'sketch1'
4. Download second sketch
5. SSH again
6. COpy again - call it Sketch2
7. CD to nice safe place
execute Sketch1 - (Sketch2 is already running because you downloaded it in Step 4)
As Arduino sketches are just normal Linux programs - this should work... YMMV