How can I combine two fields in a SelectList text description?

you could do something like this:

ViewData["accountlist"] = 
    new SelectList((from s in time.Anagrafica_Dipendente.ToList() select new { 
        ID_Dipendente=s.ID_Dipendente,
        FullName = s.Surname + " " + s.Name}), 
        "ID_Dipendente", 
        "FullName", 
        null);

Add a new property to time.Anagrafica_Dipendente which will represent the concatenation of the two properties:

public string Fullname 
{
    get 
    {
        return string.Format("{0} {1}", Surname, Name);
    }
}

and then use this:

ViewData["accountlist"] = new SelectList(
    time.Anagrafica_Dipendente.ToList(), 
    "ID_Dipendente", 
    "Fullname", 
    null
); 

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Update: As of C# 6.0, the property can be more concisely written as:

public string Fullname => string.Format("{0} {1}", Surname, Name);

Learn more about expression-bodied properties here.