i have the same issue from time to time, im looking for a perfect solution so for now i dont have one for you.
What works for me sometimes is i take the single and cant take him down so i drive forward and get the other leg and lock my hands in a wrestlers grip and change it to a double leg, usually wheni do this its kinda flashy and fast and i have got "nice take down" remarks for doing it, but it wont always work, especially if the person has a good base and you just cant reach far enough to get a hold of that second leg.
Some people can just stay on 1 leg forever and i hate that, ill drive them all around the ring with 1 leg they refuse to drop. So i see wherre your coming from.
Ive never done what iron said but i can see it working and i might try it, fall to the floor and slide your legs in between his makingthe figure four lock and going for the heel hook as i desribed here Steve-d MMA » Heel Hook, Standing Opponent