You know, I'm not sure if the body works harder (the uterus) to expell twins? I'm not sure why it would, seeing it's expelling a baby at a time. My point was, if it doesn't (of which I am not totally certain), than the only complication I can think of is the uterus stretching more, which would be the cause for concern in the pregnancy in totality. My suggestion was only a hypothetical one. I thought maybe the OP could do some research on that. Sorry if that was unclear.
I don't think a "twin uterus" is working harder, unless it's possible that by being over-stretched, the normal process of contractions and birthing are too much.
Of course, that opens a whole 'nother can of worms! Should we even be discussing it as "over-stretched"? More does not necessarily mean "too much". It's hard not to pick up and perpetuate the negative vibes that mainstream medicine applies to pregnancy in general and multiples pregnancy even more-so.