Worst case scenario is you'll have to pay an annual membership at two different clubs each year. "Reciprocal memebership" does exist but it's really aimed at the occasional visitor from another club.
From my experience, most gliding clubs are very accomodating and you're better off talking to them individually. I'm sure you'll be able to work something out.
At the end of the day, the clubs will want your membership so it's in their interest to help you out.
Good luck with it!