Hi Abi ,
Use the below code, assuming you have 2 statements, the below code will work vice a versa i.e. if user change the value in 2nd input then first will get auto fill.
Qualtrics.SurveyEngine.addOnReady(function()
{
jQuery('input[type="text"]').eq(0).change(function(){
var secondinput = jQuery('input[type="text"]').eq(1);
if(jQuery(this).val() > 0)
{
secondinput.val(100-jQuery(this).val())
}
});

jQuery('input[type="text"]').eq(1).change(function(){
var firstinput = jQuery('input[type="text"]').eq(0);
if(jQuery(this).val() > 0)
{
firstinput.val(100-jQuery(this).val())
}
});

});

SurajK This is awesome! Works like a charm- thanks man :)
Also, just wanted to see how this could be modified for a side-by-side table with numeric entries. Please see the image (yellow highlight row) for reference:

https://www.qualtrics.com/community/discussion/comment/29176#Comment_29176Use this code, this is as per the screenshot given above.
Qualtrics.SurveyEngine.addOnReady(function()
{
//First column
jQuery('.c4 input[type="text"]').change(function(){
var Nonecol = jQuery('.c4:eq(5) input[type="text"]').val();
if(jQuery(this).val() > 0)
{
var curval = Number(jQuery(this).val()) + Number(Nonecol)
jQuery('.c4:eq(5) input[type="text"]').val(curval)
}
});


//Second column
jQuery('.c7 input[type="text"]').change(function(){
var Nonecol = jQuery('.c7:eq(5) input[type="text"]').val();
if(jQuery(this).val() > 0)
{
var curval = Number(jQuery(this).val()) + Number(Nonecol)
jQuery('.c7:eq(5) input[type="text"]').val(curval)
}
});


jQuery('.c8 input[type="text"]').change(function(){
var Nonecol = jQuery('.c8:eq(5) input[type="text"]').val();
if(jQuery(this).val() > 0)
{
var curval = Number(jQuery(this).val()) + Number(Nonecol)
jQuery('.c8:eq(5) input[type="text"]').val(curval)
}
});


//Third Column

jQuery('.c11 input[type="text"]').change(function(){
var Nonecol = jQuery('.c11:eq(5) input[type="text"]').val();
if(jQuery(this).val() > 0)
{
var curval = Number(jQuery(this).val()) + Number(Nonecol)
jQuery('.c11:eq(5) input[type="text"]').val(curval)
}
});


jQuery('.c12 input[type="text"]').change(function(){
var Nonecol = jQuery('.c12:eq(5) input[type="text"]').val();
if(jQuery(this).val() > 0)
{
var curval = Number(jQuery(this).val()) + Number(Nonecol)
jQuery('.c12:eq(5) input[type="text"]').val(curval)
}
});


});

SurajK Thank you kind sir! :)