The old AM - PM rule is sufficient MOST of the time for the majority of cows/hfrs.
Here, my rule is slightly different where as I don't really care when the cow STARTED , as you will rarely see her do her first stand. I concentrate more on when she QUITS -- that will be your main concern. I never breed a cow that is still standing or even trying to ride another one.
My rule is -- on both donor cows as well as natural A-I cows, I wait until they are completely done and are tired of it all -- then count several hours -- say 4-6 ideally. I realize that some times this may fall into the WEEEE hours of morning to hit this time frame, but let me say this -- I haven't had a single worry about gettnig eggs fertilized if I am willing to be out with the donor at 2 in the morning, and it does happen.
Natural heats will vary in length considerably at times. Hfrs are notorious for being frisky as all get out -- and wil just keep on keeping on for ever some years. I don't get excited, I wait till they tell me they are done, and then consider the time to breed. I am very rarely sorry I do that.
Sure -- I only have a dozen head of my own cows, big deal rght?? Well --- I would be out there at midnight if needed on a hundred head too -- if you really want results -- let the cows be the boss -- and not the bell on the clock.
All kiding aside, RSC -- the first hfr/cow that stopped standing mid morning would be bred late afternoon or so here, then the other one may not get bred till after midnight if she didn't stop standing until 6-7 PM. Every one is different, all in all though, you could breed them both at the same time and probably be OK -- one may be getting a little late -- the other one would still be too early in my mind but if the semen was top flight, she'd make it!
Hope that helps with the Am-Pm things -- if I see one start in heat at 1 PM, she gets bred the next morning mid AM. If one is standing at daylight and is still standing at 5 PM, well, I go have supper, watch the news -- maybe even take a nap -- then I go breed her. Patience will make more babies than anxiety!!!! hehehe
Take care -- Terry