there is existing eng. glued on con. and i was wondering if anyone has inst. new flooring over old? i know it can be done, but what about warranty issues from manufac.??? thanks for input!

You will have to contact the manufacturer for an answer. It can be done if there are no moisture problems. No different than installing over plywood. I would NOT use an impermeable vapor barrier such as plastic (polyethylene) over the old floor and under the new one. A glued or stapled engineered most likely would perform fine if moisture readings of the existing floor showed it to be within acceptable amounts, about 6 to 10 % moisture content, no more.

Wouldn't you have an issue with fastener length?

Just curious, but can't the engineered just be refinished?

Maybe, maybe not. Perhaps the existing is 1/2" thick. And the new is 1/2". I'm confident a 1" staple would work fine. Maybe even a 1 & 1/8"

You are probably right since the fasteners go in at an angle, but remember that 1/2 material could have what a 3/16 wear layer that you wouldn't be nailing through? Is 1" the shortest fastener that they make for those nailers?

Yes, the T&G profiles are very different from brand to brand, making an absolute statement impossible. That is why I said "maybe". This is one of those scenarios where one needs to carefully examine the existing floor and select the right flooring to install on top. I have actually done this. I installed an engineered (can't mention brand because it is not sold here) over another engineered that was well attached/glued to a slab. I chose to glue down the new floor instead of stapling due to the very issue of fastener length and thickness of the flooring. I determined gluing would be better. I chose an acrylic urethane (Taylor) adhesive for easy clean-up and excellent performance, plus it was one of the adhesives recommended by the flooring manufacturer. The one thing I did to ensure good adhesion was to sand off the finish of the older, existing floor. I suppose an attempt could have been made to refinish the old floor. However, it had issues the homeowner did not like; such as beveled sides and ends, rotary veneer, some deep dents and a thin wear layer. So, it didn't make a good candidate for refinishing and the homeowner just wanted a new floor. Tearing out the old floor would have been more difficult than the light sanding I did to ensure adhesion. The job turned out great with no problems with the exception of the idiot plumber who re-installed the refer and had a leak in the water line. It warped the wood a little. I told the homeowner it may go back down and to let it dry out and never heard back.