In terms of meaning, if you use the singular verb with "A and B combined," you are suggesting "A and B combined" should be viewed as a single entity, like "the set A and B." If you use the plural verb, you are suggesting "A and B" should be viewed as at least two things, A and B, which are combined. 

In terms of usage, I did a search on [COCA][1] to see what I could turn up. I imposed a couple of commonsense restrictions: A and B both have to be singular elements (or I'd always expect *are*), and the noun phrases joined by a conjunction have to be the primary subject (so nothing like "sales of A and B combined are..." which relegates A and B to a prepositional phrase modifying *sales*).

I found three results for *combined are*:

> Health promotion and health education combined are ... ("A Profile of Western (USA) Higher Education Physical Education Degree Programs," Physical Educator, Early Winter 2006)

> Medicare and Medicaid combined are ... ("Eldering: Aging with Resilience," Futurist, Jan/Feb 2013)

>  Good nutrition and regular exercise combined are ... ("Exercise can minimize side effects of drugs used in cancer treatment," News-Medical.net, 4-30-2016)

Here are the results for *combined is*:

>  Government study finds Prozac and CBT combined is ... ("Patient, Fix Thyself" Saturday Evening Post, 2007 (Jul/Aug))

> A pre- and pro-biotic combined is ... ("Make Way for Mycoprotein in U.S. Food Supply," Consumers Research Magazine, 2001 (Sep))

I can't detect a significant difference in frequency from that sample. The usage really seems to vary based on how the writer is thinking of A and B combined, and not based on a universal rule. *Is* may be more common in a caption or headline (as the excerpt from the Saturday Evening Post appears to be), or if it's clear the resulting item is a single thing ("a pre- and pro-biotic combined" is a "synbiotic," a single thing. 


  [1]: https://www.english-corpora.org/coca/