There are a couple of shows coming up that I've been looking forward to auditioning for.  I am equally interested in both shows.  Previously, in similar situations, I've just auditioned for whatever one had auditions first and if not cast in that one, I've auditioned for the second one.  Well, I just found out that both shows are auditioning the same week.  This means that casting decisions and calls will be going on simultaneously. 

I've been involved in this as a director.  I can't remember the details other than that I was annoyed. I guess the bottom line is that as a director I assume that if you are auditioning you are available to do the show. In the situation you are describing you know ahead of time you may or may not be available to do the show.  If you get cast in both you are going to have to tell one of the directors that you decided not to do her/his show after all and are going to be in another show instead.  I'm not saying not to do it but you probably should tell the two directors upfront that you are auditioning for both show even though that might hurt your chances of being cast.

This is a tricky situation I've run into a few times myself.  I do a LOT of shows -- generally, I don't like having time "off", and while there's practically nothing in my town  (meaning I need to go at least 20 mnutes away), I've played with something like 25 different companies within an hour drive, so the options are usually many.

Given my track record of usually getting cast in the shows I target (>50%), I tend to keep track of the ones that I/we (my wife and I generally do these together) really want to do and schedule around those.  Early on, I had a situation where I auditioned for show A, which didn't have a scheduled callback, then auditioned for show B, which did, but that would be after the decision on show A.  I told the staff of B on my audition form that I was waiting to hear on show A -- with no indication on how they'd interact.  As it turns out, I didn't get A, so when the staff of B asked me at callbacks what the status was, it was easily resolved.

Something a little stickier has come up with my wife a couple times.  We had one time we'd auditioned for and GOT a show with roles we were dying for opposite each other, accepted, and that day, on the way to the theatre to pick up our scripts, she got a call from a non-equity professional theatre offering her a part that we'd gone to a callback for a few weeks prior.  She let the call roll to VM and later called back and declined, as (1) I didn't land a role as well; (2) It was a show she (actually both of us) had done with them previously; and (3) the show/parts we got at the CT were ones we were VERY excited about.  But if circumstances were different, she might have decided to take it and dropped out of the CT production.
The other was very recent.  She'd gone to a callback in early November for Show C, but they'd announced that the show would be cast by the end of the year (?!).  As I was out of contention, I found another audition for Show D in late November, and we both auditioned.  The whole process from audition to callback to casting happened in about 48 hours.  We both got show D, and she just mailed the staff of show C the next day to withdraw from consideration -- It was on her dream role list, but show D could have been halfway through rehearsals by the time C cast, and it would have been eminently unfair to withdraw from D that late.

That does remind me of one other incidence I had personally -- I had an audition for show E, then a callback for E, then auditioned for F, and then the next morning, E offered me the part.  I just immediately contacted the director for F and pulled out before he got to announcing the callback lists.

Point to the last one (or two, I suppose) is that I've stopped alerting directors to possible other opportunities.  They know that there are other activities (both other shows and different things) competing for peoples' times.  There's no promise of participation until you accept the part -- while I've considered what I would do if X, Y, and Z happened at times, and it's included dropping out of a just-accepted role to do another, it's never come to that, and for that, I'm thankful.