This is a slightly complex solution, but you could frame the upper trim of your existing tank with a wider lip. This lip should extend above the top of your rim, all around the tank. Similar to how the inner lip of your upper rim is, so that it can hold a glass top, or light fixture. Except, this will be used to hold another tank ontop of your existing tank. Build the island so that it fits inside the rim where your light fixture would normally go.
The tank on the top should be upside down, like it is a porcelan doll glass bell. I wouldn't suggest doing this with two 100Gal. tanks. However, two 20Gal. Longs or two 40Gal. Breeder tanks should work fine.
Make the lip that holds the two tanks together about 4 - 6 inches tall, so that you can leave a gap that is wide enough to feed the fish, and allow hoses and stuff to still be used. (See the picture)
If you want better access, find a tank with one side that is smashed, (For a discount. Remove the broken side by cutting all silicone with a knife, and have a glass cutter make you two doors. Where the glass was, place two double-rail sliders, one on top and one on bottom. Now place this upside-down on top of your other tank. The top tank now has full access to the lower tank, and to the island area. (Critter tanks are great for this, because they have thinner glass, not designed for water holding. This will translate into less stress on the lower tank.)