I've worked in sexual health. The usual wisdom is that if you definitely don't want to get pregnant, then you should use other methods of contraception, rather than withdrawal.
What's an "acceptable" method of contraception? There are lots of criteria - effectiveness against pregnancy is only one of them. Is it also cheap/free to use? Easily accessible? Acceptable to both parties? Minimal side effects? Little or no forward planning required? Withdrawal ticks all those boxes for you except the effectiveness against pregnancy.
If (either of you) really don't want pregnancy to occur, you need to seriously look at alternative methods. If, on the other hand, you (both) would be happy to accept a pregnancy if it were to happen, then withdrawal is an acceptable method.
It also depends a lot on HOW you use withdrawal. If you have a regular cycle, and especially if you're able to tell when you've ovulated, it can work very well to use condoms in the first half of your cycle then switch to withdrawal 48 hours after you're sure ovulation is over. Sperm can survive in the female tract for up to a week before ovulation, but the egg dies 24 hours after being released at ovulation.
I'm not at all sure that having sex twice (I presume you mean in the same session?) would make much difference to any of the above.
From the Planned Parenthood site:
What we do know is that withdrawal works about 78% of the time overall. But the odds of pregnancy are always higher during the 5 days leading up to, and during, ovulation — these are called fertile days. If no semen gets on your vulva or in your vagina, pregnancy can't happen — whether or not you're ovulating.
I know, I know -- dry facts are never quite so much fun as people being preachy are they?!