Remeber that FOP2 tracks devices, not extensions.
You can do it setting extra channels for a particular button, example
[SIP/101]
type=extension
extension=101
context=from-internal
channel=SIP/102
Lines are just slots for active calls, they are not really "lines". If you have more than one call going through either of the channel/devices listed for a button, they will occupy a line slot each.
Best regards,
