Scroll down in Guerilla on your scenario file, to reach the part called 'structure BSPs' (its at the very bottom). Add the second BSP.
To switch it in-game, use a command called switch_bsp command.
So, if you wanted a command to switch to the second BSP when the player walked through a trigger, it would be:
(script continuous a1
(sleep_until (volume_test_objects trigger (players))15)
(switch_bsp 1)
)
Note that BSP 0 is the first BSP, and BSP 1 is the second bsp, and so on. If you want your player to teleport to the second BSP, use the object_teleport command, but only after the bsp_switch command:
(script startup ihatethehceengine
(sleep_until (volume_test_objects trigger (players))15)
(switch_bsp 1)
(object_teleport (list_get (players) 0) [flag name])
)