Using 2 ECO for one ticket?
 

Does anyone know if I can use two ECO(E certificate) for one ticket?

Historically the answer has been no. Pretty sure that's still the case.

There have been examples of customers getting the company to combine a couple into one larger one but those are not particularly common and it takes some time to make that happen.

Three
 

Not recently, but ten years ago I stacked up three flight credits against one new ticket purchase. Did it in person at the EWR counter. Took over one hour face-to-face, with the rep calling their Houston supervisors' desk for help.

B7

I wonder how this is going to work for UA folks when they move to CO systems.

UA can presently take up to three (3) unused tickets or three (3) residual vouchers, or one (1) VDB voucher and one (1) residual voucher and apply them against a new ticket.

Are we going to lose this functionality?

Thanks for all your answers. One more question, if there is residual value on the CO E-certificate, Can I use it for on another ticket?

yep