First, let's confirm what you already seem to know: If there are two different UTM zones, there are effectively two different coordinate reference systems (CRS), and distances between points across the zones cannot be calculated.
So, you must do one of these first:
- Convert -- or reproject, as it's often called -- one point into the other UTM zone. Or...
- Convert both points into a different, third CRS.
There are many questions on this site about coordinate conversion or reprojection.
Either way, the problem then becomes how to accurately calculate distance given two coordinated points. Again, there are many questions on this site about distance calculation.
Briefly then, you can do one of these:
- Reverse project the two points back onto the ellipsoid and then calculate the great circle distance (or geodesic) from their geographic (lat, long) coordinates. Or...
- Convert the points into the same projected coordinate (N, E) system; calculate the very simple Pythagorean distance; then calculate and apply the scale factor to that "grid" distance to get the "ground" distance.
The easiest solution is probably to use one of the UTM zones, extended to include the outer point.