Serial uart port - dual connection

I suspect this may be a common question. If so my apologies in advance, but I can't seem to find any solid information.

I am currently working on a project where an arduino uno takes input from a GPS unit on the receive pin of a serial port, and sends output to a VGA controller attached to the transmit pin. I intend to do the initial development using a soft serial port, but would eventually look to move them to the hardware assisted serial port.

After the change over has been made it would be nice to be able to occasionally debug/update via the normal usb connection. If I do this will I need to isolate the VGA controller and GPS whilst communicating over the usb link? If so, does this have to be a complete physical isolation, e.g. a two pole switch?

Yes.

You could switch to upload, but probably not for debugging really - the code you are
trying to debug will be looking for GPS data and sending graphics on the serial pins,
which you wouldn't be able to interleave with debug statements unless you can switch
over super-fast!

fungus:
Yes.

Ta.

MarkT:
You could switch to upload, but probably not for debugging really - the code you are
trying to debug will be looking for GPS data and sending graphics on the serial pins,
which you wouldn't be able to interleave with debug statements unless you can switch
over super-fast!

You're right of course. A little bit of fuzzy thinking and explanation. (On my part.) There are other sensors involved, eg barometric pressure and temperature, and what I am looking for is to retain the ability to selectively work on those routines whilst the GPS and VGA are still in situ.

I'm not unhappy to retain the software serial port, however what's nagging at the back of my mind is a memory of reading here, some time ago, that it can consume almost half of the available computational resources on a uno. I'm not sure how true that is - or even how accurate my recollection is - but I'm assuming if I saw it here then it must be correct :|.