I don't think it would be realistic to expect volume production of the Model Y before 2020, and the Pickup will almost certainly come later.
We may see very limited production of the Model Y at the end of 2019. Hopefully Tesla won't make the same mistake it did with the Model 3, getting waaaaaaay out over its skis with an abortive start to production, months before Tesla was actually ready for high-volume production of the model.
Another reason why it's going to take two years, or perhaps a bit longer, is that it takes two years to build and fine-tune a new auto assembly plant. Tesla will certainly need at least one more assembly plant before it puts the Model Y into high-volume production.
Could Tesla develop the Model Y and the pickup simultaneously? While that's not impossible, it seems reasonably clear to me from Elon's various comments that Tesla is fairly far along on designing the Model Y, but just the other day Elon gave a statement suggesting that the Tesla Pickup exists only as some ideas in his mind.
As a reminder, Jay Cole has said that it generally takes 4-5 years to develop a brand new model of car. If the Tesla Pickup exists only as a bunch of ideas in Elon's head, that means we shouldn't expect it before 2022.
All just my opinion, of course.