D double T aka DTT stands for Down to Train. Which a male could say to a person of the same or opposite sex. It would be used to tell how a guy or girl is ok with having multiple sex partners in one night, not at the same time but one after another. Therefor he or she is down to train, as to run a train through her or him.
Guy 1:''damn dude, check that chick out she's so hot!''
Guy 2:''and she is so all over every guy at this party''
Guy 1:''she is totally D double T man! Lets get a line going!!''
When male twins take part in a sexual activity together.It has to be only with the two hence "Double".The "T" doesn't mean twin, but taboo.Also know as T&T-ing.The term has been used toward Brother-Sister twins relationship,But is wrong.It could also be used for males with a Bromance who treat each other as brothers.
I went into their room and Jacob and Jack were Double T-ing.