This is why even with one usb/serial converter, I recommend this:
http://marc.merlins.org/perso/linuxha/2010-06.html#Dealing-with-many-USB-to-Serial-Port-Converters-on-linux-and-device-naming

Marc
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Marc's magic has worked well for me so I at least don't have to worry about what ttyUSB the device comes up as, I just needed the code to get MH to reopen the port when things started failing. 

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   Chris