my two youngest are 2&1/2yrs apart. when i have had to, i carry my DD#2 on my back in an ergo/beco, and my DS on my front in a mei tai. i haven't done it in a while though, as it now entails carrying 60+ pounds of kiddos!
you can totally carry both in an ergo/sling combo. you can even do an ergo/pouch (i helped a mama of twins at work with this a few months back, it worked great for her!). here's how i had her do it-
*buckle the ergo around your waist/hips and let it hang down
*put on the sling/pouch, put baby in, and adjust
*finally, get the toddler on your back and then fasten the ergo, making sure that the chest slip is high enough so that it doesn't knock into the baby's head
i would not recommend putting the sling on over the ergo, as it wouldn't be as secure. basically as long as you put the front carrier on first you'll be fine!