The first thought would have been simply moving the Dell over to the secondary to test the card with that monitor only to see if that worked there. The separate attempt with the Acer by itself was to see if the monitor was still good when plugged into the primary.
Hot plugging the Acer in while the system was running may have damaged both the card and monitor somehow since live power was active. Since the Dell didn't see anything on the secondary it sounds too much like you did hurt the card! Not good!
The fact that the Acer has life on the primary is a relief as far as the new lcd is concerned. But a new card to run both is now a costly lesson unfortunately. The one last attempt would be shutting down, unplugging the Acer and see if any life is seen one at a time on the secondary with only one plugged in in order to further test the card. But be prepared for the secondary being toast!
When ever addin any new hardware the first thing is making sure the power is off and some will even turn the breaker switch on the supply off as well as unplug the ac cord depending on if the case is being opened up or not.
With usb printers the power is turned off on those before plugging into a usb port and later turned on to allow Windows to detect them. For more sensitive hardwares power off!
The only thing left now before replacing the card is simply turning it off, unplug the Acer from the primary, power up with the Dell to see if you get anything then. Most likely you did actually cook the secondary on the card.