We don't use ours as much now for mall trips since my 2 are older, but we still use it for walks to the playground. We have a Bumbleride Indie twin which we used almost daily for the first 1.5 years of DD's life. I used it mostly for walks outside (we lived in the city for a year, so it was our main mode of transport to library, coffee shop, playground, pharmacy, etc), and occasional mall trips.
If I was just running in to a store for a quick trip like the Starbucks from the car, DD would go in the snap and go and DS would walk. For a grocery store or target trip, DD would go in the baby bjorn and DS would go in the shopping cart. When DD got to big for those things, I bought a cheap Maclaren double on craigslist to keep in the car ($50). We used that a bunch for about 6-8 months (from 12 months to 20 ish months, I guess). Then I got brave enough to let them both walk into the store with me.
The problem I had once DD got older with only having her in a stroller wasn't that DS was jealous of DD - DD was jealous of DS getting to walk. So I found it easier to keep them both contained. Even to this day, I try to get the double shopping carts because if I let DS walk, DD makes such a fuss in the store so she can walk too...and having 2 kids roaming around the store is no fun! I can handle it for quick trips to the pharmacy, coffee shop, etc, but for grocery shopping or Target runs, it is much better to still have them both contained (at almost 4 and almost 2.5!