Hello, old Dev Shane here from past pwo/future pokemanz
There is a few reason's that this is not a good idea.
Sure there might be cases where its fine, you will login ok and there might not be an issue with what you are doing.. but let me note down some of the reasons you should not.
There are a few UIS issues with PWO.
1. When they start the server after a crash within 5 minutes of it happening, everyone except staff will get a UIS issue if they were just logged in, this is because they need to set everyones online variable in database back to 0. This wont work no matter how much you change your username. So this isnt the UIS issue you are "working" around.
2. UIS caused by a deadlock on a script dealing with your character, and so it can not disconnect, and is stuck in the server till a restart. This will most likely cause your account not to save. So if you do manage to work your way around this one like you said, its quite possible you will lose game progress within the last 4 minutes i think the save cycle is. And who knows what having 2 of the same accounts online will do to the server, especially when it loops through to grab your user account when someone chooses to battle you or what not.
3. UIS issue caused by the account in the server being flagged to logout, but somehow manages to be flagged as already out. There is a small hotfix that is possible to fix this issue, if the devs want a hand i can explain. but result will be same as above.
there is a few more but, not so easy to fix.
But point of my reply is. Dont do this
Same goes with adding a space infront or behind your username, if this isn't fixed yet in pwo it can cause many issues, here is 2 common ones.
1. your battles will mess up and appear you are fighting your self, or opponent can look shiny and other stuff.
2. users and staff will not be able to type your name in a command.. or click battle, trade, pm etc..
for ex /battle domt wont work because you are as "domt " in the server.