I have similar gaps and have never had a double buggy, just used a sling. Don't forget that by the time you new dc is 6 months your older one will be almost always walking (at least that is how it was with mine)
There were a couple of occasions when I made them take turns, 'you get out and walk for a while and I'll lie the baby down' Or later, when the older one got tired, putting the younger one in the sling and the letting the older one ride. (you can also get a buggy board at this stage)
The big thing is your lifestyle, How much walking do you do, do you use a car a lot or buses, do you need to be doing a lot of shopping with dcs in the buggy? Do you walk long distances? (as opposed to frequent short spurts) If you mainly use the buggy for going to the park, the older one will jump out anyway. I am a great believer in getting kids to walk, so both my older ones walked most of the time from 2.5.
Also have you carried a baby in a sling before? I have had 3 slings, first 2 borrowed and they both gave me back ache after a while, or once the baby was bigger. Then we bought a babybjorn sport, this is fantastic, was still using it at 1 year, even though she hardly squeezed in it in her snowsuit. Could have climbed everest with the baby in it.