It depends on how old your eldest will be when the next one comes along. I love my bugaboo and have kept it - even though I have three little people 3yo and under. I carry the baby until they are about 4-6 months (at least - usually longer) and used the bug for the older one. Once the baby gets to the 4-6m mark, they can go in the normal seat of the bug and a skateboard attachment works great. If you have enough slings (or varying types) you can usually work out the sleeping part if they both decide to crash at once, but it really does depend on your needs.
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