what i did is i create two function on JavaScript side and remove required attribute on each client click event of respective buttons.
i.e
function onregisterbuttonclick ()
{
document.GetElementById('txtreg').required = true;
document.GetElementById('txtnewsletter').required = false;
}
function onnewsletterbuttonclick ()
{
document.GetElementById('txtreg').required = false;
document.GetElementById('txtnewsletter').required = true;
}