We got our second pushchair when Owen was about 7 or 8 months I think, mostly for trips into London (though turned out to be useful too for us a pushchair to keep in the car so don't have to keep moving the big one in and out!). Definitely worth waiting as you'll have stronger opinions on what you need/don't need later. You definitely don't need to get everything before the baby is born. We're certainly waiting to see if we think a double buggy will actually be useful or not rather than getting one in advance. Hoping we don't need to!