I guess it depends on what you're doing.
My DD was just turning two when her brother was born and letting her walk wasn't really an option. Not because she'd get tired - she's actually pretty good like that -but because I didn't want to have to deal with her running away or wanting to stop every 5 seconds (she's easily distracted LOL).
I do have a tandem pram but I only use it for shopping - she really needs "restraining" in shopping centres!
If I'm out doing something that's fairly brief I usually put Toby in the hugabub and Emily can walk, or sometimes I put Toby in the single pram and she walks. This really only works for short outings though.
If I'm going shopping for the whole morning I couldn't do it without the double pram.