Try with below code:
string xmlData = @"<projectedincomeposition><keyacct xmlns:h="true" /><mod1code>STK</mod1code><exchangerate>0</exchangerate><coupon>0.59</coupon><isexternal>false</isexternal><maturitydate /></projectedincomeposition>";
IEnumerable<XElement> roots = XElement.Parse(xmlData).Elements();
int coupon = roots.Select((item, index) => new { elementName = item.Name, Index = index }).Where(p => p.elementName == "Coupon").Select(p => p.Index + 1).FirstOrDefault();
int maturityDate = roots.Select((item, index) => new { elementName = item.Name, Index = index }).Where(p => p.elementName == "MaturityDate").Select(p => p.Index + 1).FirstOrDefault();
if (coupon < maturityDate)
{
}
else
{
}