They're not "joining a coalition", they may just vote the same once. There's no coordination between the two. The left said from the start "this government sucks, we must get rid of them". The far right said "this government needs to agree to x, y and z or we get rid of them" (and the government did x and y but not z).

The far right are split between the fact that actually their program is very close to the government and the fact that they must look like "the opposition" for their electors.