Japt, 810 bytes
r"%d*$"Vr"%d*$" +V
Explanation
r"%d*$"Vr"%d*$" +V
Ur"%d*$"VUr"%d*$" +V # Implicit input (U)
Ur and V)
r # In the first input string,# replaceRemove
"%d*$" # # 0 or moreany trailing digits
U V # By from the first input
+V # And append the second input
# Implicit output