Can anyone do a double sonic -> shadow 1-4 then over again? Or is that already a trick?

I assume that you mean:

Sonic Double 23-12 > Shadow 12-34
or maybe
Sonic Double 34-12 > Shadow 12-34

Either way, they are both combos, not tricks. And yes, many people have already done them.

BTW, Sonic Double is now just another Shadow. Sonic Double = Shadow 2.0.

Shadow Normal 2.0 23-12 > Shadow Normal 1.5 12-34
Shadow Normal 2.0 34-12 > Shadow Normal 1.5 12-34

Still, both of them are combos, not tricks. Not new combos either.