First, I am not a dentist...
Different people respond differently to anti-anxiety medication. For some people like me, 5mg Diazepam would nearly put me to sleep (even at my most anxious), for others, it wouldn't even touch them. Most people are somewhere in the middle. You may well find that 5mg Diazepam is enough to get you through.
I think that you have a few options -- The first is that you could call the dentist first thing Monday to ask if s/he thinks that you could take 2 tablets. The second is to just take 1 and if you are still super anxious when you get to your appt, you could ask then if it's ok to take another tablet. If the dentist has recommended that you take it 30mins before your appt, it's because he expects it to work relatively quickly. The third option is to call your Primary Care Provider (not sure if you are in the US, if not, call your regular doctor) -- there is probably at least someone on-call over the weekend -- to ask her/him if there is any reason to believe that taking 2 tablets would be harmful to you.
I would not take 2 tablets without consulting your dentist or medical doctor.
Best of luck to you!