Well that's the same for any vaccination for Covid, 1 or 2 jabs.
It's new so right now real time data is being gathered. I would suggest the experts are getting muddled with, well 2 MUST be better than 1.
From the last round of data released J&J were matching AZ, Pfizer etc in terms of protection. Hence why the US and the EU approved it. I believe the UK are currently now looking at the next step of data before approval. If the rumours are true that we wouldn't get our first doses until June/July then it makes sense for us to get a bit more in.