Hehe, let's see here...
Andreeuh -- Both traits are recessive, so you'd get a fish that is neither CT or DT. All offspring would be single tail with various degrees of combing, unless the CT carried a DT gene or the DT carried a CT gene, which is unlikely since these tail types are not commonly mixed by breeders.
BettaBum873 -- Heterozygosity and homozygosity have little to do with whether a trait is dominant or not. If the trait is dominant, that trait will be expressed whether the fish is heterozygous (Dd) or homozygous (DD). However, if it is recessive, it will only be expressed if the fish is homozygous (dd), so a fish heterozygous for DT (Dd) would be a single tailed fish.
Do you mean alleles, or loci? A locus is where a trait is located on the chromosome, an allele is a variation of that trait. For example, long-finned and plakat are different alleles at the same locus so you cannot have a fish with medium length fins if you breed a long-finned fish to a plakat as it is a simple dominant-recessive relationship, but CT and DT traits are on different loci so you can have a CT DT. The only finnage trait controlled by multiple loci is caudal spread (as in the difference between a VT and a HM,) so it is not a Mendelian trait where you can just do a punnet square.
Tempestuousfury -- Co-dominance isn't an issue with fin types, but incomplete dominance is with the CT trait, in my opinion, because fish heterozygous for the trait (Cc) are often combtail.
Here are a few good links on betta genetics
:
http
/www.bettas-jimsonnier.com/genetics.htm
http/www.bettysplendens.com/articles/catview.asp?catid=864
http/www.bubblenest.net/geneticsymbols.html
I'm flattered 
) 
. You can also have any combination, like CT VT, DT plakat, etc, except for having long finned plakats of course, lol.
