The key to understanding this pairing is in the description for Coda Longa and Bicorno. Fiore says that Coda Longa is good at coming to the close play, while Bicorno is good at preventing the close play. These two oppose each other quite nicely, just by their very nature. However with Bicorno sitting at a 45 degree angle up, dangling the torso and legs in front of Coda Longa, it also shows the nature of long tempo vs short tempo wards as well.
If Coda Longa makes any attack, Bicorno thrusts. Coda Longa's long tempo attacks are a detriment attacking into Bicorno's short tempo thrusts. On the other hand if Bicorno attacks with a thrust, the long tempo nature of Coda Longa allows it to easily Exchange Points, or Break the Thrust easily. Against any other Bicorno attack, Coda Longa can counter with a powerful Fendente counter cut, or a Sottani. Attacking first with either ward is not good.
Fiore also states that whatever Longa can do, Bicorno can do. So you can substitute Longa here, however remember that Longa is by it's nature a Longer Ward, and so does not offer the same tempting body and leg targets that Bicorno does. In order to entice Coda Longa into attacking, Longa will have to offer a tempting target. This is the chief difference between these two wards, and this pairing shows that off nicely. While they both can do many of the same things, they do them in slightly different ways, and those differences are crucial to understanding the wards themselves.