I am going to the UK next month and we only have 11 days. I've been to Liverpool and Manchester before and did each of them in a day. But my wife is with me and she's never gone. Should we do more than one day in these cities and shorten visits to Cardiff or Devon?
It would likely depend on what you want to show her in Manchester and Liverpool, and if you want time in Chester.
Only you and she know that.
You must have a list of things if you liked Manchester and Liverpool so much you want to go back and take your wife.
What do you want to see in Devon (a whole county) and Cardiff?
Which is more important to both of you?
Can't comment on Manchester, but if you go to Liverpool there's the Beatles Experience museum and Magical Mystery Tour bus. The bus gives you free entry to the rebuilt Cavern Club, which in turn is right around the corner from the Hard Day's Night Hotel where you can have a nice meal or a snack/tea from the "Anytime At All" menu. Doing all those would take up the better part of one day and I can attest they are loads of fun if you love the Beatles.
Apart from Beatles there's the World Museum, two cathedrals, and more info available here
For an 11-day trip to the UK, for me personally I'm not sure I would necessarily include either Liverpool or Manchester because there are lots of other places I'd find more intriguing. As the other poster commented, it depends what's important to you and your wife.