2.4G signals over single antenna
Yes, it's possible. I'm going to assume that your "radio cards" are Wi-Fi radios or something else which must each receive and transmit.

However, you can't simply connect both radios to the same antenna as you describe. If you do that, the transmitted power from one radio will render inoperable the receiver of the other radio. It's possible it just won't work, but in all likelihood you will break your radios. Receivers are designed to handle the extremely weak power received from a distant transmitter -- not the full power of a transmitter coupled directly to it.

What you need to solve this problem is a diplexer. A diplexer is a 3-port device. One port goes to the antenna, then each of the remaining two ports goes to one of your radios, each on a different channel. The diplexer has within it filters which isolate the different frequencies of each channel, so each radio does not see the other.

Unfortunately, a diplexer with a frequency response steep enough to sufficiently isolate two 2.4 GHz Wi-Fi channels will be very expensive. A separate antenna is almost certainly a more viable solution.