Mills and Boon or Harlequin Romance Books Plot: Single father with two daughters enlist the help of a nanny. He is an architect or in some construction company She desperate to have a baby and eventually falls pregnant with his baby.

[Dee]

you might want to put some of these details in the title to help ppl - just saying harlequin/mills and boon isn't helpful

[JennAnn]

Unfortunately, a lot of romance novels have similar basic plots, so it is not always easy to identify individual stories!
But "Winning the Nanny's Heart" by Shirley Jump matches a number of the details you've provided, though the children are a girl and a boy. It's one of a series of five books called "The Barlow Brothers" , some of which are also sold together (2 in 1?) You can use "Look inside" the cover of the book on amazon and read a short section of the book, to see whether you recognise it.

If your book is not this one, I was once delighted when, by following the advice given in one of the Help posts here, I managed to find a romance novel I enjoyed as a teenager, years ago. The suggestion was to use the advanced search feature on google books. (Until then, I'd never heard of this section of Google!)