If I flip two heads with "Dual Ball" can I choose both halves of a LEGEND Pokemon?
No, because LEGEND are not Basic Pokemon, you cannot choose them with Dual Ball.
Source: Unleashed FAQ (2010-05-13), PUI Rules Team (2010-05-13)

Relevant cards

Note: All applicable cards may not be listed below.
COL1_78
EX11_89
EX14_78
EX4_72
HGSS2_72
WOTCE1_139