There's enough documented cases where we KNOW this is the case to think this is likely true for a very large proportion of them. Dick Francis is a major case in point; his wife wrote most of "his" books.
It was a joint decision by the couple to market the books this way, where she was the one who insisted on not being credited, while he wanted to credit her.
I have seen too many married couples, ESPECIALLY in the '70s and '80s era, where the wife will say whatever story her husband has concocted.
If they really wanted a joint brand, well, there's been a long tradition of using fake authorial names (the earliest example I can think of is the Bobbsey Twins) where various writers write under that brand. But they didn't do that. They used HIS name. This tells me something.
Is it possible that this was something she willingly partook of? Sure.
But it's not very likely to my mind. The story "no, really, I want my wife to get credit but we HAVE to do it under my name for reasons" is very, very thin.