Udev rule for enumerating usb serial converters

This is not Endeavour/Arch related - but I have not found any solution that works…

I have two usb to serial converters (different vendor- and product id) connected to my server. It is very important that they are correctly named to ttyUSB0 and ttyUSB1 because of one application depends on “its” converter connected to ttyUSB0. But after reboots it seems to be “by chance” which one is named ttyUSB0 and which one is named ttyUSB1. This is my last “99-usb.rules” I have tried without success:

SUBSYSTEM=="tty", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", NAME="ttyUSB1", OWNER="uucp", GROUP="root"
SUBSYSTEM=="tty", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", NAME="ttyUSB0", OWNER="uucp", GROUP="root"

The devices are named by chance… Is there a possibility to set static names to them? I already have udev rules to create symlinks which I can name ttyUSB10 (or any other unused name) - but this does not help me. One applications only works with ttyUSB0.