To link 2 computers you should have cross cable. Because Serial cable is user to connect 2 or more dissimilar devices. To connect 2 similar devices like computers, Hubs, Switches you should use Cross cable only.
In Serial cable 4 pairs of cables are their, 2 pairs for Transmitting and 2 pairs for Receiving the Data. If you use serial cable to connect 2 computers, Conflict will happen while sending and receiving the data. In cross cable 2 pairs of cable will be in a different order so conflict won't happen. So use Cross Cable while connecting 2 similar devices.