This is not easy but it can be done using Apple script and named triggers.
So first you'd create two named triggers in the "Other tab" in BetterTouchTool. One for mouse up and one for mouse down:
Then you'd assign a "Run Apple Script" action to the two finger tap:
And use this Apple Script:
tell application "BetterTouchTool"
set clickState to get_number_variable "ClickState"
if clickState is 0 then
set_number_variable "ClickState" to 1
trigger_named_async_without_response "mouse_down"
else
set_number_variable "ClickState" to 0
trigger_named_async_without_response "mouse_up"
end if
end tell
