Selling as a team is a little bit like a relationship. You always want to be sure the person you are selling with complements you, and also is able to make up for your deficiencies.
In fact that is the reason why Sales Engineers are necessary at all, to make up for the deficiency in the Salesperson to know enough technically about the product in order to sell it. The reason why SEs need a Salesperson is equally about the SE's lack of ability in commercial, negotiation and selling skills. There are probably rare cases of people who can do both, but then, if you are a good salesperson, you probably won't want the bother of having to stick around and explain technical details with techies.
So at the end of the day, the Sales staff will try to get the SE who can fill their gaps, and naturally the SE who can fill multiple gaps will be more desireable. So they will be encouraged by the Sales staff to learn all of the company's products, and be an expert in each of them.
Once someone has that aura of success over them, other Salespersons might feel that their deals are not as successful, and that with this SE, they might solve their problems. So they will gradually demand that this successful SE will help them as well. This is good for the SE if they are paid on commission and if they can choose to do the biggest and best deals, but in general, they will be torn in too many directions to make them all work.
Also it means that other SEs will miss opportunities when they are available, and possibly better positioned (either through knowledge or location) to deal with them.
How to solve this problem? Ensure that SEs and Salespeople have well defined pairings or groupings, and that the roles are well understood among the company. Also it is vital that new SEs get the training and attention they need to get up to speed and make their Salespeople successful. Its equally important that the Salespeople don't lose confidence in their SE, and that both sides feel comfortable in the relationship and optimistic in its success.