I think what you simply want to do is:
TimeSpan ts1 = TimeSpan.FromHours (12.34);
TimeSpan ts2 = TimeSpan.FromHours (56.78);
TimeSpan sum = ts1 + ts2;
Console.WriteLine ("Total timespan = {0}hrs which is {1}hrs and {2}mins",
sum.TotalHours, sum.Hours, sum.Minutes);
/ravi