Maylee, I'll be honest with ya. I have 60+ tanks set up and almost every one of them has a double sponge filter. I love them for multiple reasons, BUT they can be a pain in the a$$ because almost every week I have some popping off the glass and rising to the surface.
I think what I am going to do soon is silicone a clip or the filter itself upright on a piece of ceramic tile, and place that in the tank. Then that challenge will be over.
As far as seeding your filter, you can also take a small bucket of water, set up your double sponge in it, and squeeze out the mulm from your current sponge into the bucket. Squeeze the sponges on the double filter in the water to get them good and "dirty." Let it run for a day, and that will allow the bacteria to impregnate the double sponges.