I have this problem with one of the wrestlers I train with. I kick out his other leg and then move into a heel-hook or take side-control. If you can't lift the leg for a slam then take away his balance entirely. Often, it is just as good.
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
if you have his right leg then use your right hand to grab his right ankle and bring his leg inside, then get an underhook or wrap around his waist with your left and kick out his other leg, very effective