My beta at 14dpo was 239 and at 16dpo it was 497 and I'm having twins. Since the numbers double every 2-3 days, though, if you're off in your calculations of dpo at all, it's hard to evaluate the numbers to know if they're high or what, though. I had IVF so there's obviously no ambiguity! 800 is really high and 1300 is really, really high for 16dpo-- even for a twin pregnancy. But it's doubling at a reasonable rate (e.g., not out of control) and if you're a day or two further along than you think, then it all makes sense. Or if you had two really early implanters! I hope your ultrasounds come soon!
(I should say, though, that lots of twin moms have opening betas higher than mine-- on my twin board of other newly pregnant women I had one of the lower twin betas)