Water coming up in kitchen sinks

Any thoughts on what makes water come up out of kitchen sinks when they seem to be draining properly? The only glitch we could see is when both sinks are full of water and you try to empty both at the same time. It seems to get overwhelmed and stops draining. I plunged the kitchen sinks extensively to the point that I could smell sewer gases (I'm thinking I cleared the trap of water). Ran some water and the smell went away but the sinks behaved in the same way. The homeowner told me that she noticed the sinks backing up while someone was showering. I believe the drains for the washroom and the kitchen sinks drain to the same pipe. Thoughts? Is this simply a clog or is there some fundamental problem with the piping? Another point of information: This only just began, it's not an ongoing issue and it did happen before. She had RotoRooter in and they snaked the line with good results. She is concerned that she doesn't want to have the expense of snaking lines every year or two. One more, the p-trap is in the finished basement ceiling and it is a fixed type.

Thanks!!

It sounds as though there is a partial blockage in the drain serving the kitchen sinks, and possibly, the bathroom. If she doesn't want to have the lines snaked regularly, I would recommend that she hires someone to snake the drain lines, and then camera inspect them to make sure that the pipes are completely opened. This will cost more than just a basic snake job, but will verify the condition of the drain piping.

Thanks phishfood. I appreciate your response. I think I will recommend the camera and snaking. This is a new house for her and I think someone looking at the plumbing from the inside would be a great idea. While I have you, what are your thoughts on "hydro" snaking? We have an outfit up here that does high pressure water snaking of pipes. The theory is that it cleans the whole inside of the pipe better than a regular snake. Thoughts?