[RTTY] Com port gurus?

[Bill Turner]

I have an odd problem with com ports. I'm assembling a new computer to
replace one damaged by a lightning strike. 

I bought a new 2-port com port card and can not get it to work. I've
installed the drivers, checked for updates and done everything I can
think of. Com port numbers are correct, etc, etc, but MMTTY will not
communicate with it on either connector. Windows device manager says
both ports are working normally.

On my old computer, I had two individual com port cards, so on a hunch
I tried them in the new computer and they work fine.

I thought maybe I had a defective card so I ordered another one of a
different brand, but have the same problem. Neither port works. Tried
it in a different slot, no change.

Running WindowsXP with all updates, Sempron 2800 CPU, 1 Gig of RAM.

Why would a single port card work but a dual port card won't?

Bill, W6WRT, baffled