Are you still able to use the Kari Me with your youngest? If so, put him on your front in that and then put the Ergo on your back (straps over the top of the Kari Me) with your oldest in. I've done it with a woven wrap and an R&R so pretty much the same thing, and with a Babyhawk and the R&R.
The Ergo should be fine for your oldest for a little while but you might want to look at toddler/pre-schooler size carriers if he really likes it. I didn't bother at first when my oldest started wanting to be worn again, but after a while of being carried it wasn't comfortable enough for her so then we got a bigger carrier. I wouldn't have bothered if she went off it quickly.
If your toddler is going to be worn regularly I'd look into a toddler carrier, since it'll be used by your younger one soon as well. The toddler Tula is amazing (look up the Tula Love fb group!) the toddler dream carrier and kinderpack are both great too For most toddlers the ergo doesn't support knee to knee and is too short on their back.
There is a tandem carrying Facebook group that could help as well
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