I see different ways to solve it....
1) create your own PropertyManager page.
2) Keep the userform at top of the screen while running the macro. See this post for macro and information.
3) Request the user to have select the two points before they run macro. Do that by check what is selected and send message to user if it doesn't fit the needs for the macro, then exit the macro.
4) This idea is not at all recommended and is kind off last resort.... You could prompt the user to select the first point, have a loop looking for selections, exit loop if selected object is OK or due to time out. Repeat the step for the next point. You could skip time out but with a very high risk that the user gets stuck in the loop forever I tested this once in, see link below.
But I strongly recommend that you go for one of the other options above.