So, take the two planks, flip them upside down, use lumber store 1x2s with mitered corners and oversized holes (lengthwise oversize) to allow for movement, and use the 1x2 as a cleat. These keep the seat together and allow for movement.
Legs can be a 4" post or a 3/4 or 4/4 side panel, and can be attached by screws, dowels, mortise/tenon, or whatever. You can brace the legs with either a lower shelf or stretcher, or diagonal braces. In the event of the braces, these can be cut to rest on the inside edge of the cleats. Both the legs and the diagonal braces can be screwed to the cleats for fastening, and both will also need to be screwed (if you go that route) to the underside of the bench.
Best off: the tool list is only a circular saw or jig saw, a drill, and a couple of clamps. To build the panel for the legs, clamp together the two pieces that will be glued edge to edge and plane them smooth - square does not matter at this stage because the edges will be matching. If you opt for something other than screws, you only add a chisel to the tool list... simply drill out the holes and use the chisel to square them up. The tenons (or wedges, or whatever) can still be cut by jig saw. You might need to add a sanding block, though, if you go this route.
And, because it's live edge wood (if you can find live edge shorts from a local sawmill), it's unique automatically. (Best yet, you can still add inlays if you want to.) And I seem to recall this blog post talking about butterfly keys to hold two panel edges together to add interest.... Dale Oster, perhaps?