Should we glue above and below our rubber underlayment for wooden floors?
You’re right...we avoid two layers of vapor barriers, but that is for walls, ceilings, etc. with material in between the vapor barriers that can have moisture in the lumber or trap moisture within the void space as heat transfer the walls, ceilings and reaches it Dew Point. 

In this instance you are “sealing” the DRY rubber underlayment. (Emphasis on “dry”.) You’ll have minimal moisture movement downward into the rubber underlayment compared to air movement up into ceilings or sideways into walls. (Most of the moisture will come up from below.)

However, I’d contact customer service (in writing) and ask them to clarify...and keep a copy of their response. 

There seems to be two conflicts 1) sealing the rubber underlayment to the plywood subfloor, and 2) sealing the rubber underlayment to the engineered flooring. I like the idea of sealing the rubber underlayment to the floor, but we generally allow the wood flooring to move, depending on seasonal humidity, etc. 

I’d re-read the instructions to be certain and then I’d follow the manufacturer’s instructions...after they clarify...for your location.