Anime Anime website has an article about the Tsubasa & XXXHOLiC OADs and, according to them, all four installments will be released in four consecutive months, starting in January with XXXHOLiC's first OAD.
That means we'll have XXXHOLiC's first OAD in January, Tsubasa's first OAD in February, then one in March and the final one in April. That, according to Anime Anime.
I was under the impression that the second OAD of both XXXHOLiC and Tsubasa would be released simultaneously, but it seems like there will be an interval of one month between the releases.
Basing on that, we have two different scenarios:
January: XXXHOLiC #14 with OAD 1
February: Tsubasa #26 with OAD 1
March: XXXHOLiC #15 with OAD 2
April: Tsubasa #27 with OAD 2
or
January: XXXHOLiC #14 with OAD 1
February: Tsubasa #26 with OAD 1
March: Tsubasa #27 with OAD 2
April: XXXHOLiC #15 with OAD 2
The schedule above should not be considered as official as it is based on my own speculations on the subject. I will report here once the official dates are announced.
I think the first scenario is more likely to happen, but at the same time, it's hard to believe that between XXXHOLiC #14 and #15 there's only 2 months of difference. But CLAMP can always haste XXXHOLiC's pace in order to meet that deadline. We shouldn't discard the second scenario either.
So what about the manga? Is volume 27 and volume 15 the final volumes of Tsubasa and XXXHOLiC? Or there is a 28th volume for Tsubasa? If so, will it run for an entire volume without XXXHOLiC or is there a 16th volume planned for XXXHOLiC as well?
For the first time I'm thinking that 27 is just as possible as 28 volumes for Tsubasa. I need the official dates to make a decision xD But for now my guesses are either 27 or 28 volumes for Tsubasa and 15 volumes for XXXHOLiC.