Quick update, i figured out the problem, i did not write a tool to automate a workaround yet. But the issue is that the client seem to take an exclusive lock on its config database now (so the 2nd client cant read it and default to vhigh).
Using a tool to unlock files, there are many out here, i did not dig too deep, I'm using
UNLOCKER 1.9.2 BY CEDRICK 'NITCH' COLLOMB but be carefull, while the application itself is clean, the installer try to change your homepage / offer to install crap. You can skip all the crap on it, its not forced, but read before clicking to be safe (or just wait till out fix it in inside AB or i finalise an automated tool for it).
Basically, using a file unlocker, after running each client (except the last one no point here), you must unlock all files inside documents/Archeage/USER/udf/, before starting the next client. Be warned that this mean that if you change the config when more than one client are open, you will corrupt your option database, in which case the only fix is to delete the USER/udf/ folder.
So a step by step assuming you dont care about the original configs would be (they are likely to be lost either way)
To Set everything up:
1. Delete Documents/Archeage/USER/udf/ while the clients are shutdown
2. Start a single client and set your configs like you want them, then exit the client properly (wait the 10sec timer). This should write a few files in USER/udf a file like 000002.log and a MANIFEST along others
3. Restart your single client and double check that your configs loaded properly the shut it down again
4. Backup your USER/udf directory so you can easily restore it if your configs corrupt themself.
And then to start your clients:
1. Start the first client
2. Unlock the directory and all files inside "documents/Archeage/USER/udf/"
3. Start your next client
4. Repeat 2-3 for as many clients as wanted
5. enjoy all your clients at low.
As i said, this is only the manual process, sometime in the next few days, if OUT doesnt write a fix directly inside AB, i will release a small tool that will automate the process to start multiple clients (the setup part would still be required but thats a one time thing).
The Change in the client from a shared lock to an exclusive lock on those file, was likely an attempt by XL to reduce the risk of the option database getting corrupted, since the game doesnt seem to be able to fix it on its own (and then all your config changes reset every restart AB or not until you delete the USER/udf/ folder)