I'm going to be doing a wedding cake that will include an 8" round double height (8" tall) tier. In the past, I've simply used two slightly smaller cardboard cake boards and wooden dowels to stack the tier. But, I've recently became a believer in the SPS system...so, can I use the SPS to stack the double height tier also? I'm thinking I'd use a 7" plate under the top 8" cake? Or will it be too likely to shift? I hope that makes sense and thank you for any help and advice!

Yes, if you're talking about using two SPS sets in one 8" tall tier... it's no different than making a two-tier cake with different size diameter tiers only these tiers just happen to be the same diameter. It's good to hear that you're considering using supports for every 4" of cake height rather than trying to use 8" tall legs.

in my gallery there is an all white wedding cake with a double high 10" tier. I used two 10" plates under it. I also knew that the rosettes would hide any plate overhang so using the 10" wasn't an issue, but even without the camouflage of the roses, it would have been fine.

Thank you for the advice!