It's not confusing.
Using Fire Punch against Heatran would not work because (2)x(0) = 0. (2) Double damage for using a Fighting type on a Fire/Metal, and (0) for using Fire against Heatran's Flash Fire.
Using Thunder Punch against Gyarados would cause double damage instead of quadruple damage: (4) x (0.5) = 2. (4) for using Electric on a Water/Flying. (0.5) for using Fighting against a Flying.
Likewise, Muddy Water (Water/Ground) would cause 16x damage against Magcargo (Rock/Fire) because (4) x (4) = 16.
If you know how to multiply and divide according to the target pokemon's double types, you know how to multiply and divide for the attacking pokemon's double-typed moves.
"It's too confusing". I've seen more complicated formulas for moves on Bulbapedia. So a "confusing" battle system where second graders are not "clever" enough to figure out multiplication and division does not discount its possibility.
Alternatively, I was thinking perhaps moves like the elemental punches and Tri-attack could be divided into separate mini-attacks.
Fire Punch would be made up of:
-a physical Fighting move with 50 base power
-a special Fire move with 25 base power
Tri-Attack would be made up of:
-a special Fire move with 27 base power with 6.67% chance of burn
-a special Electric move with 27 base power with 6.67% chance of paralysis
-a special Ice move with 27 base power with 6.67% chance of freeze
Together, the attack would add up to 81 base power. Attacking against Dragons and Grass types would be "complicated", but it would be fun.
Using Fire Punch against Heatran would not work because (2)x(0) = 0. (2) Double damage for using a Fighting type on a Fire/Metal, and (0) for using Fire against Heatran's Flash Fire.
Using Thunder Punch against Gyarados would cause double damage instead of quadruple damage: (4) x (0.5) = 2. (4) for using Electric on a Water/Flying. (0.5) for using Fighting against a Flying.
Likewise, Muddy Water (Water/Ground) would cause 16x damage against Magcargo (Rock/Fire) because (4) x (4) = 16.
If you know how to multiply and divide according to the target pokemon's double types, you know how to multiply and divide for the attacking pokemon's double-typed moves.
"It's too confusing". I've seen more complicated formulas for moves on Bulbapedia. So a "confusing" battle system where second graders are not "clever" enough to figure out multiplication and division does not discount its possibility.
Alternatively, I was thinking perhaps moves like the elemental punches and Tri-attack could be divided into separate mini-attacks.
Fire Punch would be made up of:
-a physical Fighting move with 50 base power
-a special Fire move with 25 base power
Tri-Attack would be made up of:
-a special Fire move with 27 base power with 6.67% chance of burn
-a special Electric move with 27 base power with 6.67% chance of paralysis
-a special Ice move with 27 base power with 6.67% chance of freeze
Together, the attack would add up to 81 base power. Attacking against Dragons and Grass types would be "complicated", but it would be fun.