If you are on the board of a different theatre and use that only to promote your theatre performances that can be inappropriate.  But of course you can be on more than one board--as long as you don't divulge what you shouldn't and as long as you really serve each well.

 

If in doubt in this area, look to other non-profits.  Many have board members that serve several different charities.

 

Now consider a discussion that I just had with a customer who heads a local community theatre.  He and other theatres in the area developed a co-marketing venture where if you show a ticket stub from any of them, you can use that as a buy one get one free coupon for any of the community theatres.  Even the professional theatres bought into this co-marketing venture (although you can't get the pro tickets for a discount, you can show your pro ticket for a discount at the others).  This is smart for several reasons,1.) it encourages attendance with people that already decided that they like the show--in the case of a person who shows the ticket to bring a date back the second night. 2.) it brings people from related geography. 3.) it makes people feel they get a bargain without diluting the normal purchase price.  4.) it opens doors to promote in new places. 5.)it also can get contributors.  If you were on the board of other theatres and served legitimately in helping them, you could propose comarketing ventures--as long as it was in both theatre's interests.