I don't know if there's something for screws in wood, as wood's a rather strange material.
For bolts into steel, however, there's the AISC Manual (no prices listed, but expect it to be in the $300+ range); there used to be a separate book on joints, so you could calculate the strength based on the size of the fastener, bolt pattern, etc. There was also a section for calculating the strength of different weld patterns.
Now, the rules for the optimal strengthy are going to be similar -- further apart will support a greater moment, so when the contact patch is a square, you want the first two to be in opposing corners. If using four, fill all of the corners. However, because it's wood, you have a chance of spliting, so you don't want to go too close to the edges. (if you have to; pre-drill).
The other thing to remember is that with screws and bolts, the fasteners shouldn't be taking the full load -- they're pulling the structural material together, so that the load's transfered as friction ... this means if you see a gap between the two pieces, you need more fasteners. If you have a really large contact patch, drop another screw in the middle.
Paraffin is a low temperature wax (around 100 F) that is inexpensive and can be melted in a pot (or disposable aluminum baking pan) to allow dipping. Its brittle at room temperature, so your slabs must be handled gently after dipping.
It is best to dip just the end grain ends of the piece and leave the other 4 sides alone. Keep them out of the heat and sun, a root cellar would be perfect. (probably not an option in FLA). Perhaps a cooler, this will let them dry very slowly, a key to minimizing splitting.
Addendum:
It is usual to make the original piece a bit oversize and then mill it to the final size after it is dry. This is why a 2 x 4 is 1.5 x 3.5, a combination of shrinkage and final sizing.
This will allow you to trim the top and bottom so that they are parallel and will hold the vase steadily. A block plane or sander can make one side flat and a thickness planer will make the other side parallel to it.
Best Answer
Keep in mind that lumber nominal dimensions are not actual dimensions. A 2x6 is really 1.5" x 5.5" not 2" x 6" as the name suggests. So a built-up beam made from two 2x6's is actually just 3 inches wide. If you're supposed to use a 4x6 timber, you might not be able to substitute a built-up beam made from two 2x6's.
Using built-up beams made from doubled up or tripled up dimensional lumber is an everyday framing method. The most common way to build them is simply nailing them together. There are different standards for the specifics of nailing them up.
This illustration from Details for Conventional Wood Frame Construction shows nails 32" apart in a W pattern, with three nails at the ends. The document specifies 20d nails. Other standards may specify different types of nails and spacing.
It may not seem like a lot of nails, and there's no glue, but it's not necessarily better to make the beam more rigid; this type of beam may function better if the pieces are allowed to move a little so that all the pieces contact the material they support. The important thing is to stick to whatever the design intended, and that that design is something applicable codes allow.