Craftsman (2-year-old) garage door opener 139.18451 (with 2 remotes)
Craftsman (brand new) wireless keyless entry 139.18775

Followed instructions for both on "learn" button. However, it appears the motor unit will allow/learn only either the remote or keyless at any given time. In short, if I use the keyless, the remotes don't work. If I use the remotes, the keyless won't work.

Wondering if I picked up the wrong keyless set. Thanks for shedding light.

You are doing something wrong, will lean multiple and even temporary. Do you have instructions booklet.

Nib,

It sounds like you bought the wrong wireless keypad. Is your learn button red? If so you need a wireless keypad for a security plus, sounds like yours is for a green learn button.

On some, if you hold down the learn button too long, it will erase all codes(previous code), after teaching one unit, try the second. Reread instructions carefully and exactly, if it doesn't work, come back.