I installed a Bruce Lock and Fold engineered hardwood floor last spring for a customer.
The floor has since seperated in places. The salesperson that sold the floor also sold
an underlayment, that in my opinion was too thick (.125 inches) but I had no say in the
matter and since the product is relatively new, didn't know any better. Is there a maximum thickness this product should be installed over and if the underlayment is too
thick, could this cause it to seperate? I asked a Bruce technical rep. and he did not give
me a definite answer.

If the underlayment is not dense enough it can definetely cause this problem, as with any floating floor.
I couldnt find any specs on Armstrong's underlayment. But they do have "recommended" underlayments. I would imagine if it wasn't one of their recommended underlayments, they will wash their hands of it. ( I'm assuming the underlayment is definetely not from Armstrong ) ?

Sounds to me like maybe the tech.rep. figured the least I say the better , in this situation. Probably pass the buck to the dealers rep, and HE wont want to throw his dealer under the bus
That's a tough one...It's the dealer's responsibility to furnish the correct product, and it's the installer's responsibility to install the product correctly . It's both responsibilities to inform the customer. I hope you and the customer aren't victims of the dealer, trying to cut corners.

I've only heard of one manufacture that lets you do two underlayments and thats ....I forgot, lol but you have to use theirs.

Edit:Quickstep

Could definetly be site conditions, too dry...extremely low rh can make the locking mechanism fail.

1/8" (.125) is pretty standard for floating underlay, and seriously doubt that it is the issue. Where is it separating, side or end joints? Did you check the subfloor flatness and correct any issues? Any noticeable deflection (bounce) in the floor? Yes, climate might contribute, but IMO the most likely cause is movement, weakening the locking system. I took some sample material that was provided to me and was able to kick the joints apart once it was installed, so over a year, I don't have any doubt that movement could separate the locking profile.

Yes it very well could be a prep issue.

But I've seen some funky underlayments , that would result in excessive give. There's a dealer out here, believe it or not, buys huge rolls of foam packaging material and sells it for underlayment It's thin, but has no density. What was used ???

First off, I want to thank everyone that responded to my posting. The underlayment I used is made by precision, it's called silent step or something like that. The subfloor was in pretty good shape but still there could be something I missed under there. I'm from Denver and the climate here is very dry, especially in the winter, so the floor could be drying out and shrinking a bit which happens to most floors here. I will probably go in and do a moisture test on the floor. I hate this floor and I wish they would quit selling it, it is such garbage.
Thanks again for your input, Ive had a chance to view some of your work (collective) and am very impressed. I do exclusively pre-finished wood, so I haven't had the opportunity to do some of the more intricate work that you all do, really enjoyed seeing it.

Your manufacture sets rh requirements for this exact issue. Call them and ask that specific question. Besides the flatness issue which in combination could be the culprit like mentioned before.

If an inspector walks in and pulls out his thermo hygrometer and that rh is not in the zone, fahget about it, claim denied....site caused. Home owner is at fault under the maintenance clause, which everyone uses these days.

I just read the requirements for a Quickstep laminate job I am doing now and they say between 50-60 percent rh. LOL Thats impossible!

I'd like to know too..... where is it separating?