Hi Matt,
test this code
(defun c:test (/ ep pt1 pt2 rad sp ss1 ss2 sse ssx)
(setq ss1 (entsel "\nSelect a line vertical"))
(setq pt1 (cadr ss1))
(setq ss2 (entsel "\nSelect a line horizontal"))
(setq pt2 (cadr ss2))
(setq ssx (car ss2))
(setq sse (entget ssx))
(setq sp (cdr (assoc 10 sse)))
(setq ep (cdr (assoc 11 sse)))
(if
(and pt1 pt2)
(progn
(setq rad 2)
(command "_fillet" "r" rad "_fillet" pt1 pt2)
(command "_erase" ssx "")
) ; progn
) ; if
(command "_line" sp ep "")
(princ)
)