from two triangles to a square.

Support Modeling
1

Is it possible to convert to triangle faces to be one square face?

2

Delete the Edge between them and Fill the space.

3

Select the faces you want to join and press Shift-J.

4

If the triangles are coplanar, share an edge and the normals point in the same direction, alt-J while in edit mode will join them.

5

In recent blender versions, selecting them and hitting F also works.