Use this code to access single player menu in multiplayer:
D00A5060 0001
80403FF7 0001
D00A5060 0000
80403FF7 0000
D0403FF7 0001
800A4FC4 0001
D0105304 0010
800A4FC4 0000
D00A4FC4 0001
800A325D 000?
D00A4FC4 0000
800A325D 0001
Where ? is the number of players co-op'ing

Reformatted for trainer:

;	This code seems to be working but causes annoying screen splitting in menus
;	This code also fails to keep the game in multiplayer mode when loading a
;	campaign mode level

;	Menu hack
.long				0xD00A5060
.long				0x00000001
.long				0x820B6FFC
.long				0x00000001
.long				0xE0000000
.long				0x00000000
.long				0xD00A5060
.long				0x00000001
.long				0x820B6FFC
.long				0x00000000
.long				0xE0000000
.long				0x00000000
.long				0xD20B6FFC
.long				0x00000001
.long				0x800A4FC4
.long				0x00000001
.long				0xE0000000
.long				0x00000000
.long				0xD0105304
.long				0x00000010
.long				0x800A4FC4
.long				0x00000000
.long				0xE0000000
.long				0x00000000
.long				0xD00A4FC4
.long				0x00000001
.long				0x800A325D
;	Change the question mark to the number of players playing
.long				0x0000000?
.long				0xE0000000
.long				0x00000000
.long				0xD00A4FC4
.long				0x00000000
.long				0x800A325D
.long				0x00000001
.long				0xE0000000
.long				0x00000000
;	End of Menu Hack
