Focusrite Scarlett Solo vs 2i2: One Input or Two?

Not properly. You can put both people on one microphone, which means you cannot balance them afterwards and every mistake by one person is baked into the other's track. You cannot chain two interfaces on most systems without an aggregate device, and aggregate devices drift. If two people will ever speak into two microphones at once, buy two inputs.

No. Both use the same converters and the same preamps from the same generation, and both run over USB-C at the same buffer sizes. Round trip latency is a function of buffer size, driver and computer, not of input count. What changes with more inputs is what you can capture at once, nothing else.

The newer generation adds more input headroom, a gain assistant that sets a sensible level for you, and improved converters. None of that is transformative and a third generation unit bought cheaply is still a capable interface. If you already own one, this is not an upgrade worth money. If you are buying new, the price gap is usually small.

A microphone, an XLR cable, headphones and something to stand the microphone on. That list is often forgotten in the interface budget and it adds up to as much as the interface itself. Setting sensible levels matters more than any of it: the gain staging guide on this site covers where to aim, and it is the single largest quality difference for a beginner.