You are thinking well, and the City Elite 2 is certainly a quality option for an all-terrain ride and a good pushing experience. Be aware, however, that a sibling board will make the steering harder in any case, with any stroller, and, also, for a shorter mum, also. Additionally, a kiddy ride will also make the stroller experience less all-terrain, always, no matter the stroller, as it has smaller wheels and is an extra load in the rear, making you walk with shorter steps. I summed up a lot about add-on sibling board accessories here in this article, read it to know more.
Still, you can always take a baby carrier or a wrap when going hiking in harsh terrain, putting the kiddy board upwards to not have it in your way. Sturdiness-wise, this stroller should be able to withstand it generally, so you should be fine. Again, as I said higher up, the driving of the City Elite2 is very nice, but yes, it is a larger pushchair; still, after putting the handle lower, you should have a pleasant enough steering experience. Be ready that it will be harder with the board for the sibling and always a bit of a struggle.
Otherwise, I have no better suggestions for an all-terrain stroller (I'd say check the Thule Urban Glide 2, but it is less sturdy so less suitable for a buggy board, IMHO); this is actually very, very nice stroller just count on it being larger, as an all.-terrain stroller should be. Also count on the properties changing with kiddy rides, unfortunately. But a 2yo should not need investment into a double stroller, I think, if you think about a hop-on board only.