Oh WF, the waiting must be torture - I'm so sorry. I'm not sure what to say - you say you don't want to get your hopes up, but you also ask for long shots... it's cruel isn't it, that you want a definite answer - so that means you flit between wanting a yes and wanting a no (not wanting - you know what I mean - if it's a no, you just want to know). So - hopeful stories, but also stories to help you loosen your grip on the hope. I'm so desperately sorry you're being tortured this way.
This next bit is not a real life storing - just me talking - but it's a little hopeful I suppose, so it you're not in a place where hopeful stuff is useful to you, don't bothering reading it. 
Tbh, my personal opinion is that your beta hCG is pretty much unreadable, because of the two embryos. I believe that one of your embryos started to implant, and therefore would have been producing hCG too. So your 790 last Monday could have been for two embryos. If you could partial out the amount for the other embryo (which you can't) then your increase to 1450 would be a bigger % increase. It would still be less than the guideline of doubling every 48-72 hours, and at least 60% increase in 48 hours. Some people do have slow building hCG levels - and if you wanted to be hopeful, at this stage, the best you could hope to see is what you've just seen. Is there any chance of talking them into another hCG at the end of the week? Though that's pointless I guess - only a scan is going to give you a definitive answer.
You don't know how hard my fingers are crossed - and I shall be calling Petal to arrange an entire flipping field of ovine fertility sheep. Sod that - you're Welsh frank - we'll have a whole blooming Wales worth of sheep.
http://www.betabase.info/showBasicChart.php?type=Single