I fly EVA a fair amount. To ensure I book a codeshare I put in XXX City code to TPE on co.com and ensure that I am getting a CO codeshare. Then I book on EVA using my EVA number so that I can go under hotfares and save a little additional. Then I call EVA and switch to my CO number. I've only had a problem once with miles posting and it was taken care of within 48 hours by CO.