Is there a term of the trade for showing a customer a combination such as two suits, three shirts that each go well with both suits, and four ties that each go well with the combinations of suits and shirts?
I'm sorry, if there's a specific universal industry term, I don't know it. But it's a very interesting question, and I'd like to offer what little information I have from experience over the years. My suspicion is that it's more likely vernacular terms would evolve that would be unofficial and differ from chain to chain, store to store and change over time.
As a shopper, I have seen advertising and sales programs with terms such as suite, combo, pre-coordinated, and ensemble to suggest items that could be purchased and worn together. Such programs have had rather short histories, rarely continuing for more than a few years, suggesting they may not have been very successful in the intent to sell more goods. The illustration below is from an ad for a retailer named Contempo that appeared in the April 1937 issue of Esquire. They employed this as their form of marketing, and appear to only have been around 10 years, or so.
This process used to be the province of the men's wear salesman, and as such, was dependent upon their level of experience, talent and taste. I.e., good ones were often passionate and did it well, nubes, not so much. But I'm afraid in the current retail environment, I would expect only limited and questionable assistance. If you can find otherwise, you might wish to cultivate it.
Edit: I just realized that another concept might be helpful for you, that of the personal shopper. Some larger stores offer these, and the good ones are very skilled. Though it's usually higher-end retailers, and the merchandise they offer tends to be pricey. And there are also individuals that do this on an independent basis, and are commonly compensated by the retailers for merchandise their client purchases.