Let \(\mathcal {R}_q=\mathbb {F}_q\times (\mathbb {F}_q+v\mathbb {F}_q)\) with \(v^2=v\) and q be an odd prime power. Firstly, we give two Gray maps from \(\mathcal {R}_q^n\) to \(\mathbb {F}_q^{3n}\) . Next, we provide the structure of all \((1,1-2v)\) -constacyclic codes and their dual codes with a length n over \(\mathcal {R}_q\) . By means of these structures, we give also the structure of all Euclidean hulls and sums of \((1,1-2v)\) -constacyclic codes of length n over \(\mathcal {R}_q\) . Finally, using Steane’s construction and quantum construction X of the Euclidean dual, we provide two methods for constructing quantum error-correcting (QEC, for short) codes via the Euclidean sums and hulls of \((1,1-2v)\) -constacyclic codes and linear codes of length n over \(\mathcal {R}_q\) . To enrich the variety of available QEC codes, some new QEC codes are constructed to illustrate our results.