I always sort of envisioned it as they come out of the shells, combine, and then the shells allow them to functionally be both separate AND combined at once.
Wow, those are awesome...and even BIGGER, but proportionally not that different from the originals in scale. Checking out both Double Pretender Megatron and Prime, the only actual Pretenders I have now, and memories of Groundbreaker and Octopunch from earlier in life, I realize what was meant by the shells always being larger, and I'd just honestly overlooked. Being custom-made for a smaller toy the shell's still twice the height, and while Megatron recycled the Grand Maximus shell, I have a G1 Encore Cerebros I use with it and it's still quite large comparatively.
I can only imagine how much those FansProject shells must have gone for, much less the combiner itself. I was hoping for 'smaller/cheaper' for the Cores, but...maybe not. I guess I will wait and see.
Edit: Never mind on the imagining, I found it easier than I expected...$60-$80 EACH at the start. With the combiner about $300. Yikes. It's an odd day when I can look at something and realize I'm literally not that crazy. That doesn't happen often.