Is it correct that one is able to use this reserve space as storage at
the price of missing in-place recovery?
Yes. You will be able to keep up and running while S2D rebalances itself using reserve space. Theoretically, this can save you from data loss in case you wasn't able to replace failed drive in time, and the second drive failure occurred after S2D finished rebalancing. Unfortunately, I didn't manage to find any detailed description of this feature and it's impact on the number of drive failures S2D can tolerate. The chances of loosing your data after the second drive failure are still pretty high.
Your disk layout will be as follows:
3x 1TB SSD per node: contribute into usable capacity.
1x 2TB SSD per node: will be used as reserve space.
You are looking at nearly 3TB usable capacity in such case.
P.S: I can't disagree with @BaronSamedi1958: 2-node Storage Spaces Direct is a weak setup. I don't recommend using it in any serious production.
There are alternative solutions that work better in 2-node configurations:
- StarWind VSAN Free - works like a charm in 2-node environments, can be deployed on a bare-metal Windows.
- HPE VSA Free - a good piece of software as well, but runs only inside a virtual machine, which leads to performance compromises. 1TB limitation in free version.