I don't see how they could do it and actually expect to receive the money. A lot of jw families contain an unbelieving mates. It's doubtful that an unbelieving mate, male or female, would accept their spouse giving 10% of their income to the WTS.
Plus as others have said most dubs don't have much money so why upset your followers with tithing when you won't really get much out of it.
The WTS is probably better off just beating the money drum more regularly than they used to. Show some emotionally charged videos and look really sad. The money will come in from those that have it.