The whole sense of an inter-lab trial is to compare the results of the same method run at both labs. And the sample prep is an inherent and important feature of each method. Sometimes probably even the most important feature, running the HPLC method itself often (not always, unfortunately) is rather easy, but with sample preparation there might be a lot of pitfalls.
Running the same samples at both labs will basically only show that both labs are able to run the HPLC method and evaluate the raw data correctly. It does NOT show that both labs are able to handle the whole procedure and produce equivalent results. So, no, this is definitely not acceptable. This would be a procedure to validate only the HPLC method - you want to validate the whole analytical method, and the HPLC is only a part of it.
Preparing one set of samples and running the same samples at both labs can be one method of troubleshooting if you actually see that results from both labs (with individually prepared samples) are different. But it's definitely against the sense of an inter-lab trial.
Concerning related substances, you definitely should be able to produce reproducible results at the specification limit; I mean, this is the concentration range where the serious stuff happens - is the results out-of-spec or not?
Things can be quite different when looking at lower concentrations. A customer once wanted us to check reproducibilty at both the specification limit and the lower limit of quantitation - at was completely stunned that reproducibility was significantly worse at the quantitation limit. What a surprise.
