You perform for a table. If another table watches, then thats ok. If you're worried about not being able to repeat your tricks to the neighboring table, don't. Chances are, you won't perform for each and every table consecutively. Normally if you are performing for tables, you will be peforming after they have placed their order and before their food arrives. With that in mind, the wait staff will not normally be doing a lot of checking up with the table. "Is everything alright"..."No!, we don't have our food!" Get it? The idea for the restaurant is to get'em in and get'em out. You are not there to keep them there...you are there to entertain them while they wait and to possibly help influence them to come back with friends or to tell others about the experience to get them to come in. You should keep your routines somewhat short in length. That way if the food shows up while you're performing, you can quickly wrap up.
As for tables being too close, like I said before, that shouldn't be a problem with neighboring tables. However, I performed at one restaurant (Max & Erma's) where the tables were, not only, close together, but the isles were realy narrow. That made performing very, very difficult. I was difficult to the point where it wasn't even fun.