Hey palley,
I will start off by addressing your second question first. NI-VISA does not have a problem working with multi-serial port cards. NI sells several devices with multiple serial ports, which all work with NI-VISA. However, it is possible that your card implements things differently than the native serial ports. NI-VISA relies on your serial cards supporting all of the functionality found in the built-in native serial ports on your computer. If you can give us more information about what is not working and perhaps some NI-SPY logs of you trying to run one of the example programs we might be able to find a work around.
To address your first question, I would like to see two NI-SPY logs of you running a very simple LabVIEW and .NE
T program that are performing the same operation. It would be best if the applications were wrote so that one port is opened, then you write once, read once, and then close the port and exit the application. We can then compare the two applications and see what is happening. It would also help if you could provide the source code for the two.
I hope this helps out. Perhaps you will be able to tell between the SPY captures what is happening. Otherwise, just post the information (spy files and source code) to the forum and we can try to help out.
-JoshuaP