Johan, for the Monsters of Melodica recording of “Spain” the reason it sounds like two voices is simple, there are two melodicas playing. A Vibrandoneon and a Suzuki Pro 37 on the lead melody. For the solos, we used a Hammond Hyper Melodion with a chorus pedal by (CHASE BLISS AUDIO “WARPED VINYL ANALOG”) and delay pedal by (MOOG “MOOGER FOOGER 104M”) and finished off with plate reverb. Note: We used these pedals because We did not like the chorus plugin they were using and the outboard rack equipment had issues. You do not have to use these specific devices, you can use other pedals and plugin and experiment until you find the combination of sounds you like.
Trial and error can be your best friends when it comes to sound effects.
Monsters of Melodica