Whilst my younger two are 19 months apart (little older than your two will be) i found having DD#3 in the sling, and i had DD#2 in the pram. DD#3 would sleep almost all the time when she goes in the sling, allowing me to chat to and, focus my attention on DD#2. For the short amount of time you can spend at the shops with two young ones ......i doubt your DD is going to feel left out - KWIM/?/???