Good question. You'd think he'd be aiming for Angel Eyes, since him and Blondie are "friends". However, Tuco, as his name suggests, is the "ugly", AKA hes very opportunistic. It wouldnt surprise me if his plan was to let Blondie shoot Angel Eyes, then quickly shoot Blondie, and take the pot for himself. Of course, the joke would be on him, since Blondie wrote nothing on the rock. Im 90% sure he was going for Angel Eyes, but theres that 10% wildcard just because Tuco is unpredictable.