dodrio (evolution of doduo) and magneton (evo of magnemite)
you don't
I don't know soz
yes, when it evolves into dodrio.
no it can not.
Cross Chop isn't a TM, just an attack some Pokemon can learn.
its not a TM but dodtrio can learn it by leveling up.
In Pokemon I've never heard of that attack Could you mean: Double Kick or Tri attack. If you mean Tri-attack the well: Tri Attack does damage, and has a 20% chance of either paralyzing, freezing or burning the target. Each ailment has a 6.67% chance of being inflicted. The Pokemon that learn it is: Duduo, Dodrio, and all the Porygons! So i think you meant Tri attack.
Some Pokemon may learn it or try going to the move tutor which is in a little west from fallarbor town.
Magneton can only learn Tri Attack through the Move Relearner in Pastoria City in Pokémon Diamond, Pearl and Platinum.
Level 21
you don't
I don't know soz
yes, when it evolves into dodrio.
Slash, Rock Slide, Earthquake, Tri attack, Magnitude, Rock Tomb, Sand Tomb, dig.
tri attack is not a TM in platinum. tri attack used to be a TM, and could be found on the roof of the celadon department store, but future generations replaced it.
this is not all of them. tri attack, sand attack, scracth, growl, magnitude, dig, mud slap, sand tomb, slash, earthquake, and fissure
There is no best level to evolve Magneton into Magnezone as long as it knows Tri Attack before you evolve it if you want it to know Tri Attack because Magnezone cannot learn Tri Attack in any way because other than Magneton learning Tri Attack via the Move Re-learner, their move pools are exactly the same except for a Move Tutor being able to teach Magnezone the Iron Head attack which is a move that Magneton cannot learn.