Ok I think I can help you out! Did you say it was a German couple? cool!
Your plan to do it in one piece is good, I am guessing that you are going to have to butt together two pieces anyway, because unless there are any sources of 24 foot pecan lumber…. it might be difficult, I am not certian on the available lengths of pecan What you could also consider is using a what we call a "ship" pattern of putting the boards together, so stagering the boards, so you have a row of 2 boards than a row of three or something in that manner. I think this would work great. Have you asked them if they wanted it solid, because veneer might be easier to handle and to seam up togther, depending what kind of set up you have in your shop. Not to mention of course the availablity of pecan veneer.
If you do do it in two pieces 8 feet long I could recommend joining the two 8 foot sections with each other using a tongue and groove method, that way the table can expand (as it will be outside) and contract with the weather. The table you must keep straight with the expansion and contraction by the use of sliding dovetails or perhaps even steel rods, or bars that are set into pockets to keep the table straight, just keep track of where they are and you can drill and tap out threads to screw into and attach the legs or base/strechers of the table to that. That is how I would do it.
These would be the "text book " solutions that we would use here, but….
Keep in mind also Biergartenbänke (ein bank, zwei bänke, see German is easy), that we have here are usualy made of economical (I do not like to use the word cheap because they are sturdy) wood (pine, fir…) and are attached underneath with steel legs and the top is screwed to this steel folding legs with carraige bolts (the ones with the rounded tops) so a person can see the heads of the bolts from the top of the table while enjoying beer. Usually people do not care because it is often covered with a table cloth, but sometimes not…. the main event is usually the beer (especially during Oktoberfest).
The cross piece you could attach in the middle with biscuits but only in the middle say 8 cm, other wise such a wide piece in the outside could eventually start to move and cause the splitting you mentioned. If it must stay flush on one end, you could attach it there with dowels or biscuits, and perhaps some sort of "false tongue" that could fit into a groove cut into both sides, without glue, and if you wanted to make it easy, you could forget the biscuits and just apply glue for about 8 cm on of the ends or in the middle, depending on what your needs are, so it would stay fixed, the table would stay straight on the end where it is attached and still move along the "false tongue" with moisture changing.
I would also be interested in what kind of bier they are serving there, you know, I think I might have another one now, its the right time of the evening!
I hope I could help! Und bitte sagen, "Grüß Gott, aus Bayern!" or "Servus!"