I've dped several times as both a top and a bottom, so I can definitely give you some tips. As with sex in general, the most important thing is that the bottom is well prepared and feels very relaxed. Do not go for the gold right away. Both tops should take time to get the bottom worked up and fuck him individually. One dick needs to be able to go in very easily if you want to be able to get two in there. Poppers will be majorly helpful for the bottom.
I agree with what others have said about the positioning. It's true that typically one top will be more passive, but they don't have to be. If both have pretty long cocks, you can get both tops thrusting in this position without anyone coming out. There are also other options for positions. The two tops can both lie on their backs facing opposite directions, put their cocks together, and then the bottom can sit on them and ride them as one big cock. Another decent option is for one top to fuck the bottom doggy style and then have the other top (who needs to be somewhat limber) get between them so that his butt ends up in the other tops stomach/chest area and then he inserts just above the other top. The good thing about this position is that the tops are unlikely to push each other out and it's easier for both tops to actively fuck the bottom.
The size of the dicks is important, but this does come down to what the bottom can handle and is used to. Your cock looks big but not monstrous. If you can find someone a similar size or smaller, you'll probably do well. I have tried to dp bottom for two very girth guys and it was a struggle. On the other hand, taking two smaller cocks can actually be less filling than taking one giant monstercock, so it's not necessarily a huge struggle, just depends on the sizes.
My last piece of advice is to be patient and be prepared to have fun even if the dp doesn't work out. If it doesn't work out the time, just consider why it didn't work and learn from those mistakes. Keep trying different positions. Don't give up after one failed attempt. If ultimately it just isn't going to happen, that's ok. It's just not always successful, so you don't want to set yourself for too much disappointment if you can't get it to work out. You can still have a great time tagteaming.
You also asked what it feels like - incredible is the answer! Dp as a top is the ultimate form of frotting. You get to feel another top rub their cock back and forth against yours inside a tight hole. If done well, you will find it hard not to cum pretty quickly. For the bottom, it feels like just what you'd imagine - being utterly consumed by an enormous amount of cock, which is exactly why it's so amazing as the bottom.
Please let us know how it goes!