I know when I switch my adapter to another usb port, I get another serial port #, but when I plug two adapters in. (in different ports of course) I can not get read the second adapter. its says the software failed to install right, please help
What I think is happening is due to USB enumeration. Since I don't know which USB-serial adapter you're using, or would even be familiar with it, I can't say exactly.
So the simple answer is both serial adapters are identifying themselves with the same identification information on the USB line. When you add the second one the OS sees duplication and cannot address it, which means no communication.
I have no issues myself when using multiple IOGear adapters.