The phrase "an equal interest" is correct and usable in written English.
It can be used when discussing shared stakes or involvement in a particular matter, often in legal, financial, or collaborative contexts.
Example: "Both partners have an equal interest in the success of the project."
Alternatives: "a shared interest" or "an equal stake".
Now Washington finds itself, to India's distress, with an equal interest in both countries.
"We formed the best committee; there's no chairman because everyone has an equal interest in the project".
The book ferries between thinking and dreaming, a strait navigable only by poetry, which, even this late in its development, keeps an equal interest in both.
It doesn't matter if you're a big tourism business or a small tourism business: you've all got an equal interest in what's being done".
And their commitment to juvenile humor (the stage banter on their live album can be summed up in two words: "dog semen") camouflaged an equal interest in the evergreen pop topic of adolescent melancholy.
So when females dispersed to neighboring groups, males from their home community could recognize their daughters or sisters, together with their in-laws, who had an equal interest in their children's welfare.
Among the impressive younger players are Blue Note-signed trumpeter Ambrose Akinmusire, and drummer Tyshawn Sorey, who has an almost equal interest in modern classical composition.
As new therapies are required across all types of cancer, it may be that clinicians specialising in all tumour sites have a broadly equal interest in research, because of the constantly emerging treatments in their field.
A company makes two loans of equal amounts at equal interest rates to a partner.
Mayer gave no thought to parenting as a loss of self or a threat to creativity; she recorded with equal interest the frustrations and the revelations of motherhood.
