There are two good ways to take out 64:

T16 — D8 = 64

T8 — D20 = 64

This is a really nice out. On your first dart you should aim at the T8 — T16 wire. Either one will leave you a nice double. If you miss into the S8 you have S16 for D20. If you miss into the S16 you then can aim at the S8/S16 wire for either 32 or 40.