I have the twin axle set up and the physical compressor is no more powerful than the single axle one. The difference is in the solenoids.

I got my ramp on eBay. Search for “dog car ramp”

Make sure you get a folding one that has grip on the walkway. They gain their rigididy from being folded out. You will most likely have to teach the dog to use it. Mine soon got the hang of it by putting treats just out of reach at the top end.

The X5 tailgate is not good for dogs. Mine have both caught their back legs trying to jump in, pulling the lower tailgate upwards into their belly. I had the same issue with a Range Rover. They do this once and they do t won’t to get in the car again.

The ramp folds in half then sits in the back with the dogs.

Good luck.

Do let us know how you get on.

Bruce