How to concatenate two columns of strings and insert \_ between them, please?

I have a file:

```
L  o     45
OK kklkj 5

```

The desired result is:

```
L_o      45
OK_kklkj 5

```

I tried:

```
awk '{printf "%12s %22.2f\n", $1+'_'+$2, $3}' file

```

which converts strings to numbers. Thank you.