one is tx both rx
In most of the cases one of these is used for tx and both are used for rx, but one at a time. This is diversity reception. If you look at the PC card near the antennas you may see holes for a coaxial socket. If you do this is where to run the tansmission line for your new antenna from. There should be some pads at the end of the trace where a capacitor which goes to one of the other antennas can be moved to go to the coaxial socket tracce.
If there is no such set of holes you can cut the trace to each existing antenna and solder coaxial cable to them and run this to your new antennas.