The resource "Old Testament Quotations and Allusions in the New Testament" sets them side by side and allows navigations starting either from the NT or OT use the TOC. As can be seen from the illustration below, it is possible to specify the version it will use to display text. However, there is only one version setting. In the example below, I choose the NA28 to see my NT text in Greek. But of course, the NA28 does not cover the OT. So OT passages show up in English. The reverse happens if I choose a LXX as my version: the OT passages will show in Greek but not the NT, as below:
I suggested first the possibility of having two settings and perhaps it would still be useful. There is however a way of doing this. It comes from realizing that the English text is based on your preferred Bible. So, all one needs to do is hit the home page, and change it temporarily to the needed Greek text. In the example below, I kept the NA28 in the resource and changed my preferred Bible to the LXX:
Of course, the fun is not limited to Greek. One could compare the MT and Syriac gospels for instance using the same method...
