The concept of multi threaded application software and computer processors running at many millions of instructions per second means that for all intents and purposes, the processes in the machine (cameras, mounts, guide camera, etc) run concurrently, with no interference between them. Your guide cam and main cam can download simultaneously with no delay /lag to other processes. The computer processor(s) switches between the processes to ensure they are serviced effectivley.
'Simultaneous' is not entirely technically true and depends on many things, but the huge processor and bus (internal data transfer ) speeds mean the tasks you need to perform in terms of astronomy gear (mount, camera, guide camera, rotator, dome and shutter robotics, add anything you like to this list) means that your computer's capablity is barely taxed. The amount of downtime the processor has is huge (even when it is simultaneously downloading a hi res image and a guide cam image) - you can see this for yourself on a Windows computer by running Task Manager and looking at the performance tab.
USB connectors can be daisy chained if you don't have enough on the laptop, just buy as many usb hubs as you need. I use two four port hubs to connect eight items of gear to my old (2001 vintage) Dell with 4 GB RAM.
Going back to your dual core laptop. My Toshiba dual core with 4 GB of RAM ran Sitech, PHD2 guiding, image capture software, my dome control robotics, and played youtube videos simultaneously.
you won't have any computer capacity problems with a single computer running all your gear.
good luck with it.