Run equal-length string from both markers to just past the side of the grove that is closest to the markers. Label the line on the right side as line A, and the line on the left side as line B
Run a string that is parallel to the line between the markers. Label this line C.
Measure the angles between corners AC and BC. Adjust the lines as necessary to make both angles 90 degrees. When the angles match 90 degrees, then line C is perfectly parallel with the line thought the trees between the two markers.
Look for a gap in the woods that allows you to run a straight line D that is equal in length to A and B back into the woods from line C. Line D should have a right angle at corner CD.
Place a marker at the end of line D. This marker is along the straight line stretching from marker to marker.
Add more lines similar to D and stretching into the woods, each with its own marker at the end. Each straight line terminates in a point along the line between the two boundary markers. When enough markers are established it will give an indication of the path of the straight line between the markers.