How should we specify multiple channel values in a button configuration?

[SIP/600]
type=extension
extension=600
context=from-internal
label=John Doe
mailbox=600@default
extenvoicemail=*600@default
channel=IAX2/600,SIP/600.2

or

[SIP/600]
type=extension
extension=600
context=from-internal
label=John Doe
mailbox=600@default
extenvoicemail=*600@default
channel=IAX2/600
channel=SIP/600.2