Given the global and unconditioned craze for multifin boards, it can be useful to try and give some extra information on the topic.

Twin and multi fin rush more or less exploded when a single rider, Kauli Seadi, a couple of years ago, outplaced world cup competition with his innovative and aggressive sailing style. As often happens in sport, when one cannot blame it on the athlet, it blames it on the equipment (unlike soccer there are no coaches to be dispensed of their job here :-) ). In short, more or less all top WC athletes but one started to compete in waves on twins.

The twin was really appreciated by manufacturers to revitalize the design and the market for wave boards, that had been pretty much static for years. In this it played and still plays a vital role. 

Twin (and subsequent perversions of the original idea, born with the illusion of trying to do even better under the rather simplistic idea of "the more the merrier") besides some - actually - pretty defined advantages in some precise conditions - not very typical of your home spot ( perfectly side to side-off, not too big) also inevitably carry a certain number of disadvantages.

They are slower. Much slower. In a sport that makes getting on a plane its main raison d'etre, this is not insignificant. 
As demonstrated with clarity and simplicity in this article in order to have the same lift area as in a single fin board, a twin must carry 1.33 times its total fin area. 

Now the surface of fin (more precisely, its reference area) is directly related to the drag that it produces (ie the force it produces in resistance to forward movement). Since fins excise a large influence on the overall drag of the fin+board system table (by heart at least 50%, but maybe someone is able to spot specific studies on this) here we obtain from an increase of 33% of total fin surface area at least 15% in reduced top speed (and slower acceleration +) 
Is it worth it? You decide.

In South Africa where pressure on your sail is fairly abundant or for the final heats of a wave World Cup competition in Punta Preta, maybe. 
What about Joe the-Average-Week-End-Sailor who gets less on a plane thus catches less waves?
Incidentally it is also explained why the twin fin set up is able to point better into the wind.
A 33% increase in total fin surface area implies some good extra keel area. 
And as we are taught by our yacth sailing cousins, in upwind ability the keel (alongside with the shape of the rails) is everything.

Image property of mauiultrafins