Kuki is correct, whatever the highest level is on one you will be given that level on the other. They do not combine points and you have to call the line to get a CC number or an RCI number and then have it applied to your reservation. Call the Captains Club number and give them your RCI number and you will immediately receive a Captains Club number.
Celebrity gives upgrades however they are only inside to inside or outside to outside. The CC upgrades do not apply to veranda, Concierege, Aqua Class or suites. Any other upgrade given is not a Captains Club upgrade.
Yes, you will automatically be given the equivalent status, which I believe is Diamond. Just call and you will be given a number by them and apply it to your next reservation on RC. The two credits now in a sense becomes irrelevent.
Although you will be given the equivalent status, Royal Caribbean just announced a revamping of their program. Heres a link to the press release.
I am Diamond Plus with RCCL and am getting ready to do my first Azamara cruise in April. I will be at the Discoverer level with them, which is the highest since I am at the highest with RCC (the old highest anyway).
LIBERTY OF THE SEAS - 8/28/16
LIBERTY OF THE SEAS - 9/18/16
HARMONY OF THE SEAS - 10/23/16
I've never had to notify either line about new points. It's in the computer. After my 6/09 cruise X notified me that I had moved up to Elite and soon after, I received a notice that I had moved up to the equivalent level on RCCL too, even though my last cruise on RCCL was 6/06.
Just so there is no confusion to the OP, you do have to give either cruise line your relevent number when booking. They will automatically give you the points but as the OP asked if you are not a member you have to initally set up your membership with the other line.