Which speakers, if any, combine the strengths of Harbeth and ProAc ?

Both very good speaker manufacturers. 
Unfortunately, your question is dependent upon the listener's ears.
For me, the answer is Vandersteen.
B

Probeth Model 3 (Anniversary Edition)

What are the strengths of each brand that you are looking for?

I've owned 3 Proac models and subsequently 3 Harbeth models over a period of about 30yrs.
I've owned ATC for the last couple of years.
While they have all been fine speakers, I've found ATC to offer superior timing, dynamics and broadband neutrality built around engaging and realistic mids.  
For me ATC are the more flexible, in the sense of sounding confident and convincing with any type or genre of music - classical/jazz/rock/electronic etc - which suits my eclectic music tastes.

Rockport and Verity Audio come to mind.