Welcome and Congratulations!! Tandem Babywearing is the practice of "wearing" or carrying more than one baby at a time in either a special tandem carrier or each in their own carrier.
The main carrier styles are Buckles or Soft Structured Carriers (SSCs), Meh Dais/Bei Dais or Asian Style Carriers, Ring Slings, and Wraps (either woven, hybrid, or stretchy).
Buckles are least intimidating. Woven wraps are most versatile. Meh dais and ring slings are a little of both.
I personally recommend the wearer should be comfortable with and quite proficient at front and/or hip carries with the carrier before attempting back carries; otherwise on back for full term babies with no health concerns, you can use a woven wrap up high from day 1, a Meh Dai up high from the day baby is comfy legs out and the carrier does not need modification, a ring sling and buckles once baby sits unassisted. http://rachelgaston-pifer.blogspot.com/2016/03/when-can-i-start-back-carries-in.html
Most carriers can be used for tandem wear. Whichever carrier(s) or carry(s) you are most comfortable wearing one baby at a time with is good to start with tandem wearing. Whichever baby you most often wear on your back is who should go on your back. The top edge of the carrier should fit snugly across baby's upper back to prevent baby from interfering with your sense of balance. With back wrap carries and meh dais it can help to tie off under bum in back, at shoulder or on your side. For two in one wrap it is more secure to knot off the back carry first, tied at shoulder (TAS) gives more options than tied under bum (TUB), spread passes cause less digging than gathered passes.