Now for what won't work -- dating both of them
at the same time. That might get complicated, and will
probably invoke anxiety and jealousy. You will inevitably
find the newer relationship more exciting at first (it's
human nature). And if you get physically involved, that
will create an even bigger mess... How can you
honestly and objectively sort out your feelings with
the complications of an intimate relationship? That
is too dangerous. Keep things on a friendship level!
Regardless of which one you choose, you need to be
very careful that you understand what you are feeling
for this new guy (D). It is probably just infatuation.
It may be curiosity or the temporary excitement you
feel over having someone pay special attention to you.
(And I doubt you could truly love him, after only one
day spent with him and a bus ride.) Whatever it is,
you need to seriously examine your feelings to try to
figure out if it is merely a crush or if there is something
deeper there, which will only truly develop over a longer
period of time anyway.
So while I am not sure what you should do, I do know
what I think you shouldn't do: you shouldn't sneak around.
You shouldn't base an important decision on feelings
of infatuation or a "crush" which often fades
over time. And you shouldn't jump into a relationship
before you have had time to develop a solid friendship
and really get to know who this person is.
One final thought: it sounds as though T has some serious
problems he needs to work out in his life, ones which
will affect your relationship. On the other hand, D
had not formally declared his feelings for you, so you
don't know if he is even interested in dating you. So
slow down and enjoy his friendship. In time, you will
know what his feelings are, and then you will also know
the direction your other relationship with T is headed.