Hi there - I have the same setup as you so can help as no-one else has replied.
Yes - you use the same SD card to do the update, plan routes with homeport and update the plotter.
1 - download the update - I do it from here:
Even though I'm pretty techie, I find the Garmin site not that intuitive to use.
2 - put SD card into plotter and update (it may do this automatically)
3 - then put SD card back into your PC to work with homeport. Homeport won't have the maps without the SD card being inserted.
Also, I make a duplicate SD card and keep that in a sealed watertight bag in the boat, just in case the first one it gets damaged / lost. And try it to make sure it works.
Hope this helps