
If it was me I would go down the separate present route, I'm sure they're having a joint party for ease but maybe individually they'd like their 40th to be special. Maybe you would give us a bit for info on the couples, what they like, where the live etc..
Some quick ideas though:
Female - spa package, maybe a massage, facial, mani/pedi at your local beauty salon - maybe you would even join her make it a bit of a girls afternoon out with a nice lunch too
Male - a bit harder but is he into any sports? perhaps you could try & get him 2 tickets for a sporting event maybe a local cricket match, again your OH could go along too if he fancy's it.
With those ideas they will get individually treated, do something so the other one can look after the kids & you too get to do something fun!