JSFiddle - React, Tailwind, and code Playground

HTML

<input id="txtdebito" type="text" />
<input id="txtcredito" type="text" />

CSS

input[readonly="readonly"]{
background:whitesmoke;
}

JavaScript

window.onload = function(){
    var deb = document.getElementById("txtdebito");
    var cre = document.getElementById("txtcredito"); 
    
    deb.onblur = function(){
        if(deb.value !== ""){
            cre.disabled = true; 
        }
        else{
            cre.disabled = false;
        }        
    }
    cre.onblur = function(){
        if(cre.value !== ""){
            deb.disabled = true; 
        }  
        else{
            deb.disabled = false;
        }                
    }   
}