Are babies sharing membranes? Surely not, and surely therefore his birth would not be more dangerous than a singleton with a velamentous insertion? (i'm genuinely asking, i'm not a midwife!). Where are the vessels running? Is it slightly velamentous or severely so? Will his membranes rupturing be likely to tear vessels? By what process exactly does the doctor fear the cord will be torn?
In my uneducated opinion the cord insertion probably isn't more relevant than it would be for a singleton, and most singletons with this issue don't show up until after birth because they don't routinely scan singleton pregnancies for that sort of thing.
I had a baby with a marginal, velamentous insertion (i think that's how one would put it - placenta was heart-shaped, cord inserted at one edge with vessels going along the membranes for 1-2cm before inserting) and a true knot 6 weeks ago and all was fine because my midwife just sat there while we got on with it. No-one ruptured anything, or pulled anything. Obviously that is more or less useless anecdotal rambling!