While I would not suggest keeping the animals within 6'' of each other, they can coexist in the same tank. For the most part the LTA's and Clams come from similar habitats, nutrient rich sandy lagoons, and thereofre would be well suited to the same tank.
I would suggest a 6" sand bed with lots of small rock rubble. Regarding the lighting, it is possible to keep them under PC's, however upgrading to a Metal Halide, acclimating slowly of course, would be a good call.
I'd love to see pictures when you get things settled in!