Your probably find it painful having them both in the same orifice specially your anus what can be painful with just one if your not use to it.
What you see in porn movies has taken the females a long time to get to that stage and normally a lot of stretching to be able to perform those scenes.
It's your body and if you don't feel comfortable doing dp or even having his best friend join in just tell him no he can't force you as that's bordering on rape if he does.
If you enjoyed this I suggest you take it to the next level and attempt double vag penetration. As the name implies that is when both males enter the females vagina at the same time, my wife and I have done this on numerous occasions and she ensures me that she absolutely loves it, she squirts at least 4 times when we do it. She also suggest that if you enjoyed the dp you might want to ask you bf if he has another friend and try
Something called airtight, this is when your bf and his friend are dpi g you in the doggy position while the 3rd friend stands in front of you and you suck his penis. My wife is a huge fan of this
With a little practice and willing partners, they guys can learn to not only pleasure the woman and themselves, but the other guy too!