#Java 8, (削除) 80 (削除ここまで)(削除) 69 (削除ここまで)(削除) 67 (削除ここまで)(削除) 66 (削除ここまで)(削除) 65 (削除ここまで)(削除) 64 (削除ここまで) 63 bytes

(a,b,l)->{for(;l-->0;)if(a[l]>=b[l])b[l]=a[l]*(a[l]>b[l]?1:2);}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-5 bytes thanks to @OliverGrégoire (one byte at a time.. xD)
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with 2 float-array and integer parameters and no return-type
 for(;l-->0;) // Loop over the array
 if(a[l]>=b[l]) // If the current value in `a` is larger or equal to `b`:
 b[l]= // Modify the second input-array:
 a[l]* // Use `a` multiplied by:
 (a[l]>b[l]? // If the current value in `a` is larger than `b`:
 1 // Multiply by 1
 : // Else (`a` is smaller of equal to `b`):
 2) // Multiply by 2
 // End of loop (implicit / single-line body)
} // End of method

Java 8, (削除) 80 (削除ここまで)(削除) 69 (削除ここまで)(削除) 67 (削除ここまで)(削除) 66 (削除ここまで)(削除) 65 (削除ここまで)(削除) 64 (削除ここまで) 63 bytes

(a,b,l)->{for(;l-->0;)if(a[l]>=b[l])b[l]=a[l]*(a[l]>b[l]?1:2);}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-5 bytes thanks to @OliverGrégoire (one byte at a time.. xD)
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with 2 float-array and integer parameters and no return-type
 for(;l-->0;) // Loop over the array
 if(a[l]>=b[l]) // If the current value in `a` is larger or equal to `b`:
 b[l]= // Modify the second input-array:
 a[l]* // Use `a` multiplied by:
 (a[l]>b[l]? // If the current value in `a` is larger than `b`:
 1 // Multiply by 1
 : // Else (`a` is smaller of equal to `b`):
 2) // Multiply by 2
 // End of loop (implicit / single-line body)
} // End of method

#Java 8, (削除) 80 (削除ここまで) (削除) 69 (削除ここまで) (削除) 67 (削除ここまで) (削除) 66 (削除ここまで) (削除) 65 (削除ここまで) 64 bytes

(a,b,l)->{for(;l-->0;b[l]=a[l]<b[l]?b[l]:a[l]*(a[l]>b[l]?1:2));}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-4 bytes thanks to @OliverGrégoire (one byte at a time xD).
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with two float-array and integer parameters and no return-type
 for(;l-->0; // Loop over the array
 b[l]=  // Modify the second input-array:
 a[l]<b[l]?  // If the current value in `a` is smaller than `b`:
 b[l] // Use `b`
 :a[l]>b[l]? //  Else:
 a[l]* // Use `a` multiplied by:
 (a[l]>b[l]? // If the current value in `a` is larger than `b`:
 1 // Multiply by 1
 : // Else (`a` is smaller of equal to `b`):
 2) // Multiply by 2
 ); // End of loop
} // End of method

#Java 8, (削除) 80 (削除ここまで) (削除) 69 (削除ここまで) (削除) 67 (削除ここまで) (削除) 66 (削除ここまで) (削除) 65 (削除ここまで) (削除) 64 (削除ここまで) 63 bytes

(a,b,l)->{for(;l-->0;)if(a[l]>=b[l])b[l]=a[l]*(a[l]>b[l]?1:2);}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-5 bytes thanks to @OliverGrégoire (one byte at a time.. xD)
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with 2 float-array and integer parameters and no return-type
 for(;l-->0;) // Loop over the array
 if(a[l]>=b[l]) // If the current value in `a` is larger or equal to `b`:
 b[l]= // Modify the second input-array:
 a[l]* // Use `a` multiplied by:
 (a[l]>b[l]? // If the current value in `a` is larger than `b`:
 1 // Multiply by 1
 : // Else (`a` is smaller of equal to `b`):
 2) // Multiply by 2
 // End of loop (implicit / single-line body)
} // End of method

#Java 8, (削除) 80 (削除ここまで) (削除) 69 (削除ここまで) (削除) 67 (削除ここまで) (削除) 66 (削除ここまで) 65 bytes

(a,b,l)->{for(;l-->0;b[l]=a[l]<b[l]?b[l]:a[l]>b[l]?a[l]:a[l]*2);}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-3 bytes thanks to @OliverGrégoire.
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with two float-array and integer parameters and no return-type
 for(;l-->0; // Loop over the array
 b[l]= // Modify the second input-array:
 a[l]<b[l]? // If the current value in `a` is smaller than `b`:
 b[l] // use `b`
 :a[l]>b[l]? // Else-if the current value in `a` is larger than `b`:
 a[l] // use `a`
 : // Else (both are equals)
 a[l]*2 // Use `a` times 2 (instead of `a+b`) to save bytes
 ); // End of loop
} // End of method

#Java 8, (削除) 80 (削除ここまで) (削除) 69 (削除ここまで) (削除) 67 (削除ここまで) (削除) 66 (削除ここまで) (削除) 65 (削除ここまで) 64 bytes

(a,b,l)->{for(;l-->0;b[l]=a[l]<b[l]?b[l]:a[l]*(a[l]>b[l]?1:2));}

Modifies the second input-array instead or returning a new float-array to save bytes.

-11 bytes by taking the length as additional integer-input, which is allowed according to the challenge rules.
-4 bytes thanks to @OliverGrégoire (one byte at a time xD).
-1 byte indirectly thanks to @Shaggy's JS answer, by using a[l]*2 instead of a[l]+b[l].

Explanation:

Try it here.

(a,b,l)->{ // Method with two float-array and integer parameters and no return-type
 for(;l-->0; // Loop over the array
 b[l]= // Modify the second input-array:
 a[l]<b[l]? // If the current value in `a` is smaller than `b`:
 b[l] // Use `b`
 :a[l]>b[l]? // Else:
 a[l]* // Use `a` multiplied by:
  (a[l]>b[l]? // If the current value in `a` is larger than `b`:
  1  //  Multiply by 1
 :  // Else (`a` is smaller of equal to `b`):
 2) // Multiply by 2
 ); // End of loop
} // End of method