JSFiddle - React, Tailwind, and code Playground

HTML

<select id="cities0" onchange="upDateValue(this)">
    <option value='china'>China</option>
    <option value='germany' selected>Germany</option>
    <option value='usa'>USA</option>
    <option value='russia'>Russia</option>
</select>
<select id="cities1" onchange="upDateValue(this)">
    <option value='china'>China</option>
    <option value='germany' selected>Germany</option>
    <option value='usa'>USA</option>
    <option value='russia'>Russia</option>
</select>

JavaScript

function upDateValue(v)
{
    var value=v.options[v.selectedIndex].value
    var temp,thisId=v.id;
    for (i=0;i<2;i++)
    {
        temp="cities"+i;
        if (temp!=thisId)
        {
            updateDropDownBox(temp,value);
        }
    }
}

function updateDropDownBox(id,value)
{
    var dropDownBox=document.getElementById(id);
    for (i=0;i<dropDownBox.options.length;i++)
    {
        if (value==dropDownBox.options[i].value)
        {
             dropDownBox.options[i].remove();
        }
    }
}