dd1 was 2.6 when dd2 was born. i was determined not to have a double pushchair, which was a shame because dd1 did not have much stamina - i was too busy hating being pregnant when i should have been training her to walk long distances, so the training had to start after dd2 got here.
before she was born we did lots of talking about how big sisters hold onto the pushchair, and so she did that quite often 'like a big sister' when it was still her pushchair, and if we ever saw any other big sisters holding on to the pushchair i made a big deal of it.
once dd2 arrived i had a policy of never going anywhere without having a way of getting dd1 home - she got a scooter (although she was rubbish at it for ages, in fact she pushed it quite a lot of the time, although, bizarrely it did speed up her walking a bit), and then i always had a buggy board or our ergo carrier. but i emphasised that the buggy board was for the way home when she was tired, or if we were rushing for a bus/generally in a hurry. this was part of the walking training...
i tried never to offer the buggy board, no matter how slowly she was walking, she had to ask for it.
then after we were on holiday last summer i couldn't be bothered with the buggy board any more - i was out of the habit cos we didn't take it, and her stamina seemed better, so since she was 3.3 she has just walked.
sorry, this is long, but in summary i'm so glad we never got a double pushchair. i know it depends on the age diff, but with 2.5 years for us it was absolutely not necessary.