Use MailBuilder class and AddAttachment method multiple times:
MailBuilder builder = new MailBuilder();
builder.Subject = "Hello";
builder.Text= "Both files attached.";
builder.To.Add(new MailBox("to@example.com"));
builder.From.Add(new MailBox("from@example.com"));
builder.AddAttachment("c:\\a.txt");
builder.AddAttachment("c:\\b.txt");
IMail email = builder.Create();