I am sorry but it is really hard to understand what you have written but from what I can make out there seems to be a pipe running from the fuel tank to the new external pump which is feeding the engine. the pipe which is used is using the same whole as the original fuel line accept this is longer and bigger in diameter so not to starve it.
Your question is that can use the both the fuel pumps external and intank simultaneously and also be able to use one of them as required like a backup for the other if one fails?
1st of all the internal fuel pump is called intank fuel pump and the external is Inline fuel pump.
Yes they can be used simultaneously but is a waste as you would be feeding much more fuel to the jets (carburator or injector) which would be a waste but you would get get response from your accelerator
You cannot have one shut off and the other being used, techanically you can but it would be restricting the other. So unless both are working in hand.
What you said or explained is a bit dangerous and as @Xulfiqar explained would be starving the pump of fuel during turns , brakes and acceleration, as the fuel would sloshing around the tank. The reason it does not happen with the intank pump is coz it lies at the lowest part of the tank and utilizes channels to store enough fuel.
let me explain the fuel pump a bit with some pics
This is how the whole fuel pump system looks like in the tank, accept the metal part above the round circle stay out of the tank and the remaing is inside the tank hence the intak fuel pump name. Note the fuel pump is missing which sits right where the bubble wraping is and you can see the wiring and the fuel line connection to it.
This is how your intak fuel pump looks like, the plastic sock is the superficial filter to keep big particles out of the line and this is the pick up point or the suction point of the pump and notice how flatly it is layed out and hence reducing fuel starving unless your tank is almost dry.
The top part of the fuel pump as view from the top of the tank, the first metal part is where it sits on the tank and connected. Notice the black rubber small line to the feed line.
Closer look at the top of the fuel pump.
This is how it looks when installed in the fuel tank.