Scubamom,
From your description of the foundations they appear to be PLB foundations. Their twin foundations may be manufactured by a different company than their queen foundation. Could be the reason why the top is not covered by a piece of corrugated cardboard and there is 9" spaces between the slats. I calculated ~7" spaces between the slates on my queen foundation, still way too much for a heavy latex mattress. Try contacting a PLB CSR about this.
Correct me if I’m wrong, you are saying that each twin foundation has a center head to foot support; unlike my queen foundation that has 2 head to foot supports (not counting the side supports), neither of which is in the center.
Let me try to clarify this. If each twin foundation also has a center head to toe support then, including the side supports, each foundation would have 3 head to toe supports, 6 total for both. My queen foundation has, including the side supports, 4 head to toe supports.
So we are back then to what type of steel frame you have supporting the foundations. This is the steel frame I’m currently using. I purchased it before I got the PLB foundation assuming that the foundation would have a conventional center head to toe support. Due to PLB’s unconventional foundation design the head to toe supports rest on the cross rails, not the center beam. The frame though is heavy duty so I’m not getting any sagging. With your foundations this type of frame would possibly work well since 1 side of each foundation would rest on the center beam.
Is your current steel frame strong enough to give proper support to the sleeping area of the foundation??
If I knew then what I know now I would have purchased either this foundation or this foundation without a steel frame.