The product/material that you are considering to engineer, has already been engineered and been successful in the market. Three such examples are these ones which I found in a Google Images search, where you can see many more.
From a materials engineering perspective, no new material needs to be designed, since the device can be constructed using materials that have already been engineered, and just making $2\times$ as much of it. The maximum cost $C_2$ of making the double-sided device, in terms of the cost $C_1$ of making a single-sided device, would be:
\max (C_2) = 2C_1 + \epsilon,
where $\epsilon$ is the overhead cost of having to glue together the two slabs of material. In reality though, the manufacturing cost might be much less than twice the cost of making one such device, because instead of making two separate devices and attaching them together, the whole two-sided device can be made simultaneously and possibly save some overhead costs.
- There is minimal engineering work to be done, and the materials-design portion of the project luckily doesn't require the design of any new type of material, since the most important material for such devices was already designed and can be re-used here (dry erase boards are typically made of the organic compound melamine).
- The total cost of the melamine for the product will be proportional to the number of square cm of material desired, which will be at most double (in the worst case) the cost for one-sided devices, and in some cases the cost will be exactly the same as before because we would just avoid covering the the back-side of the melamine board. These costs are worth it, to the extent that the product is already on the market and people purchase it.