Given the way that ultrasonic cleaners work, and the nature of the waves, I'm not sure that this is quite true. This sounds like someone coming from an audio background, and thinking in terms of a speaker projecting sound waves in one direction. Most ultrasonic cleaners are going to be a bit different from that.
If that were the case, then cleaning anything else in one would only result in cleaning the two outer sides of the object, however 3-D objects placed in the cleaning tank get cleaned all over, including inside various nooks and crannies.
I wouldn't want to stack my LPs right up against one-another, but I'd bet if you leave even a reasonable space between them, they will all clean quite nicely.
Caveat: I'm no expert on ultrasonics and I've never actually tried, this, so I make no claim of true understanding, here.