Separate names with a comma.
It seems kind of fixed with Parallels 15 ¯\_(ツ)_/¯
Currently on Mojave 10.14.4 Beta (18E174f) and no flicker. But it will probably not last ¯\_(ツ)_/¯
No reliable workaround for me except this silly applescript which first focuses the Control Center window and then switches focus to the VM...