You thought about the future with the Easywalker Harvey2, didn't you..? 😊 I mean, you say it yourself. Your older daughter does ride in her Easywalker Harvey2 stroller still quite a lot. To make her stand all the way for quite long walks with a newborn, all the while you'll be short of space for legs and struggling up curbs... I don't know.
What I would do is to wait. You do have the right pram if I understand correctly (not going to change it, are you..?). A sibling board is a thing that you can buy even in November, and you can, in the meantime, check where the double - tandem stroller configuration second seat is available to get quick enough... And watch your little one - maybe, but this is highly unlikely, she'll "grow out of a stroller" and won't want to sit in anymore... then buy a sibling board, MAYBE (or maybe it's not even necessary).
Otherwise, I would go with the tandem seat still - to any Harvey, the Harvey3 and Harvey2stroller tandem extension will fit mutually - and a sibling board can be quite a burden if used regularly/every day, more so even if you have a lot of curbs around... Read more in our guide about sibling buggy boards ad how - or why not - to choose one to be clearer about the topic. I would only recommend a sibling ride if you'd use it around shops and/or occasionally... for full-throttle everyday walks, I would think about a second seat more...
or, in my case and a 1.5yo when the second was born, I had a little rocket that didn't want to sit in a stroller so he walked... and, with my second being 2 and a third one born, I was using a baby wrap and an ergo carrier the most, swapping the younger and the older in the stroller system. So these I'll also recommend as alternatives to think about.