I want to make multiple blocks be placed in a /fill command in Minecraft

Fill commands only has one parameter to specify what to fill with:

fill <x1> <y1> <z1> <x2> <y2> <z2> <TileName> [dataValue] [oldBlockHandling] [dataTag]

where <TileName> [dataValue] ... [dataTag] define the block. You can specify block's data value and NBT parameters to choose block subtype (slab material, orientation, glass color, etc.), but it is impossible to fill one volume with two different blocks. (How would the game decide between them for each block in the volume, anyway?)

It is not possible to do it with /fill but you can do it by using this:

/summon FallingSand {Block:iron_block,Time:1,Passengers:[{id:FallingSand,Block:redstone_block,Time:1,Passengers:[{id:FallingSand,Block:activator_rail,Time:1,Passengers:[{id:FallingSand,Block:MinecartCommandBlock,Command:{setblock ~2 ~ ~ dirt}}]}]}]}

Or look at this if you want something more professional.

Please keep in mind that I am typing this off of memory, don't tell me off if anything is wrong, just say what is wrong in the comments, and I will fix it.

Do /fill then both of your coordinates, 50 wood, 50 stone or any other block. It will be random but both of the blocks will be placed.