No, the FPS will for sure not be doubled.

Aside from that I think it's a clear hit/miss thing with SLI/crossfire/whatever:

Sometimes it shows, sometimes it hardly affects the product at all.

I would expect the biggest benefit from having different tasks for both GPUs, where not much sync and memory transfer is needed. E.g. physics on one, graphics on another. But there are problems, making even this unpractical:

You can not assume every player has 2 GPUs.
For those that have (likely iGPU of CPU), utilizing that iGPU slows down the CPU because of shared memory bandwidth and also increased heat.
Those that used SLI or Crossfire in the past, either realized it's not worth it, or are left behind from both GPU vendors and game devs, because neither wants to support this anymore.

There were rare exceptions in the past which indeed achieved almost 200% scaling with some games, but just remember ‘micro stutter’ issues, or think about power waste.