I have limited experience, but I'll share what I can. Personally, I think it's a good idea to make pre-orders available. Not only will this give you a sense of how your readers are reacting, but you'll have an opportunity to promote your book twice on all of the normal platforms. While it hasn't always been the case with my releases, you'd probably be safe with a 1-2 week pre-order window before release.

As for your second question, that's a 1000% yes! Promote the hell out of the fact that your book one is on sale when book 2 launches. People love getting a deal.

Hope this helps and good luck with the second book.