[ОлегА]

так?

function change(_this) {   
    s = _this.options[_this.selectedIndex].value
	s = document.getElementById('select'+s);
	_this.style.display = 'none';
	s.style.display == 'none' ? s.style.display = '' : s.style.display = 'none';	
};

<select id='default' onchange='change(this);'>
<option value='1'>1</option>
<option value='2'>2</option>
</select>
<select id='select1' style="display: none;" onchange='change(this);'>
<option value='1'>1</option>
<option value='2'>2</option>
</select>
<select id='select2' style="display: none;" onchange='change(this);'>
<option value='1'>1</option>
<option value='2'>2</option>
</select>