Crossrail 2 have said they intend to submit their Hybrid Bill before the end of this Parliament (ie before 2020). If 2b is going to be submitted in Autumn 2019, then they will have to deal with both at once.
Getting enough MPs to agree to spend 2 years on a Select Committee for hearing Hybrid Bill petitions is a real challenge for the Whips. Getting twice as many to cover two Select Committees might well be impossible, which is why I suspect that one will be introduced in the Lords. Probably whichever is less controversial (possibly HS2 by then?)
One of the policy statements about HS2 was that it would continue the rate of spend for Crossrail 1, and not require a massive increase in public financing.
Forgetting parliamentary time, there must be Treasury constraints on how these mega-projects are going to be financed.
I don't think it's a given that Crossrail 2 will just "happen" when the bill gets through parliament, nor HS2b/3.
Brexit might also have an impact if the economy slows.
This is also why the government wants some private (Chinese?) financing for HS2, so they can start other projects.