Hey Everyone,
First off am a huge fan of Multiplicty and have been using for a while now working very well, but with some lag issues when switching and most of all only able to copy small files under 1 gig pretty much. So after spending a long while trying to solve it.. well I gave up lol and just kept going with it. But in this time of covid when looking for ways to pass time I came back to this and said lets give it another go.
This is my setup as well because I am sure this may vary depending on setup.
I run an old AMD 8350 8 core as a storage/server machine with Win 10 pro
I am running an AMD 3900 12 core machine as my main with Win 10 pro.
I am not running wireless, these two computers sit together here and I am connected directly to my modem with both machines (not sharing a connection the modem has 2 ethernet ports on it)
So on my 8350 I did not do a thing I left it as is and it is the secondary computer in my setup (multiplicity is now installed on the secondary machine first).
On my Main computer I wiped my drive and reinstalled windows, now normally I setup windows using the offline account method and not connected to the net while installing windows. This time I used that same offline account method but left the net connected so I could let windows see the other computer on my network with Multiplicty installed and running as well. Then went about my install, drivers updates and I made sure to install Multiplicity before my antivirus (before I had to make all kinds of setting changes to allow Multiplicity through my av and such) I am using Bitdefender TS as my AV and Malwarebytes. So this time having that all done and installed first... I had to make no changes or fix anything to allow Multiplicity through either of my AV apps. I just plugged in my codes and whamo, bamo! no issues at all and I did not have to make even one setting change.
Now the reason I want to say this method is I have seen in other threads issues with copying files and really only being able to do smaller sized files. Well I was just able to copy a 46 gig file with no issues and man was it fast I understand my connection was responsible for the transfer but still, then I tried a bigger file of 65 gigs and no problem at all and then another and another large file without even a hiccup.
So I hope this may help someone solve any issues they maybe having, if your looking for some time to kill you never know 
Thanks Stardock, stay safe everyone and stay well 