You could donate to charity in their name for a cause you know they both believe in?
My sis and BIL are both pilots on good incomes and donated to royal flying doctor service for example
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I very nice card obviously and if you dont feel comfertable not giving something just buy something small or a momento, maybe a nice photo frame?
We was invited to a destination wedding this year and were asked not to buy anything so we did not, we also had other close friends going and they all did the same thing so that helped. If it was close family I would probably still give them a gift. Good luck