Can shadowclone get assault +8 if I attack one battlefield, conquer then ride the wind to the other battlefield?

Yes, this works. Here's how.

First combat at Battlefield A: The Shadow Clone attacks (gains Attacker designation), triggering "When I attack" — you banish a unit from your trash to give it Assault 4 this turn. Combat resolves, you conquer Battlefield A. Combat ends, designations clear, but the Assault 4 effect persists until end of turn.

Ride the Wind to Battlefield B: With the turn in Open State after conquering, you play Ride the Wind. Since Battlefield B has enemy units, your move applies Contested — you are the Attacker. The Shadow Clone gains the Attacker designation in this new, separate combat, so "When I attack" fires again. You banish another unit, granting Assault 4 a second time.

Per Rule 807.2, when a unit is granted Assault by multiple sources, the values sum: Assault 4 + Assault 4 = Assault 8 while the Shadow Clone is an attacker at Battlefield B.

Couple of things to keep in mind:

• You need a unit in your trash to banish for each trigger — that's two banishes total.
• The Shadow Clone has to survive the first combat (Combat Cleanup heals it, so that's usually fine).
• Each "When I attack" only fires once per combat (Rule 383.4.e.2.a), but since these are two different combats, the second trigger is legal.