All normal precautions are doubled.
Take your time or at least take a deep breath!
Become that caregiver who always checks themself out in reflective surfaces and take 100 selfies a day!
FOR BEST SUCCESS: 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 Mei tai / meh dai up high from the day baby is comfy legs out, a ring sling and soft structured carrier (buckles) once baby sits unassisted.
1. To avoid dropping baby when putting baby on your back, establish your back loading method (hip scoot, santa toss, superman toss are all popular) and use it exclusively. Practice with a doll/sack of flour/sugar or have a spotter and practice over or near a soft surface. Use a mirror or record yourself with a set up selfie camera, so that you can see what you are doing.
2. To avoid dropping baby while wrapping, secure baby with one hand on baby or by trapping the top wrap rails under your neck or in your teeth and by holding passes between your knees.
3. To avoid dropping baby while getting SSC and MT/MD* carriers over baby, secure baby with one hand or the shoulder strap of the carrier, or put baby and carrier on front and swing around to back.
4. To avoid dropping baby when getting baby off of your back, establish your back unloading method (hip scoot, lower to thigh/soft surface, etc. are popular), practice it, and use it exclusively.
5. To avoid seams busting open, especially on older carriers, when tightening a back carry and if carrying more weight than the carrier has been tested for; always check the seams on your carrier and know what weight it has been tested to.
6. To prevent baby from arching himself into objects and possibly out of a stretchy wrap, don't use a stretchy wrap on your back.
7. To prevent babies with legs froggied from leg straightening and pushing their way out of a rigid carrier, do not froggy legs in single pocket wrap carries and single pocket carriers like SSCs, MT/MDs, and RSs.
8. To provide a secure fit, ensure that baby’s knees are outside the panel of a structured carrier; do not use the carrier's insert in a back carry or other devices (like a blanket) in a back carry
9. Unless your carrier's seams have been specially reinforced for tandem carries, putting another carrier through sewn on straps is considered an emergency (zombie apocalypse) carry as the sewn on straps’ seams can rip away making you risk dropping both babies.
10. To protect the delicate airway of babies under four full term months, do not use a structured carrier on back with a baby younger than four full term months.
11. Baby’s airway needs to be VISIBLE, wear baby high (with cheek in nape of your neck) and straight - with airway: nose, mouth, chest in a straight line; wear baby tight to prevent slumping and carrier up to the ears to secure head. Learn more about airway issues, here: http://www.sleepingbaby.net/carseats.php
12. If baby has a respiratory complication, even the common cold, use extra care in monitoring their airway.
13. To prevent a sleeping baby's head from flopping, use a scarf or shorty to secure it to your back. Just make sure you can still monitor airway and that the scarf is not across baby's nose and mouth.
14. To keep from squishing back baby when adding a heavier baby on front make sure that the front carrier is not overlapping the back carrier in a way that will squish baby or interfere with the back carrier (some carriers to avoid in this situation are one wrap for two babies, a front /back twin carrier); it is usually helpful to set up the front carrier, add back baby and carrier, and then add in front baby and tighten up the front carrier.
15. To avoid banging baby into things and prevent baby from grabbing unwanted items, use the first few weeks of back carrying to learn what your new back space feels like and when possible, wear arms in, especially for grabby big kids.
16. To prevent a carrier from exhausting baby's immature torso muscles and pushing the spine into an unnatural position, don't use a structured carrier until baby can sit assisted (and use the same rules that you would for other assisted sitting devices) and be careful when using subsequent wrap passes that you keep baby's back curved properly.
17. To avoid baby leaning out of loose top rails, grabbing unwanted items, disturbing your center of gravity making it hard for you to move, and launching themselves into unwanted destinations, do not back carry with a too large pouch sling or too short MT/MD or SSC; if using a wrap or ring sling, keep top rail snug and no lower than baby's under arms; you can use a tie off (like knotless, slip knot, sling ring) that allows later tightening.
18. To prevent your back carry from coming loose when using one woven for a front/back carry, use a back carry that allows you to easily tie a single knot before beginning your front carry.
NB: when using two carriers on back (this section is not comprehensive because this situation is considered an emergency/zombie apocalypse carry) the highest concern is baby drops from caregiver's body
1. if babies aren't sat deeply into the carriers they could pull themselves out of the carrier
2. it is very likely that one carrier will put pressure on second baby's carrier trapping baby's airway against your body and/or causing pressure points on baby
3. having a second carrier in each baby's foot vicinity, gives them a stepping stone for pulling themselves out of the carrier and you no arms to catch them with
4. getting babies into and out of the carriers is a significant drop risk
*MT/MD = mei tai/ meh dai
RS = Ring Sling
SSC = Soft structured carrier