Hi,
My wife and I did the same side by side cruise your doing last year a lot of fun and first time for us. Carnival uses a few different terminals during docking days so it’s hard to tell which one you will return to? As far as getting to the celebration after the horizon cruise you will be on your own, the cruise line doesn’t make any special accommodations for this.
We waited until 930 ish before leaving horizon and walked to the terminal F gave porters our bags and started another 7 day trip awesome.
I will caution you that it is VERY crazy busy with trucks, buses cars coming and going so be careful outside moving to next terminal there are no sidewalks on the dock side and the opposite side of street had some walk ways but some were closed for construction. We enjoy lunch the first day at Pig and Anchor restaurant it’s free and nobody there, unlike the buffet which is a mad house. Enjoy