Now, I have tested a lot and therefor an update from my case:
I really do not understand this bug, because of the current different "solutions". And where this behavior technically comes from.
Is it synced via "iPhone Cellular Calls"-Option, because some of you wrote, that it stopped happening if you switched of only this option. For me, this didn’t help. It is switched off, but the sync is still there. So this is not the final solution, but I could help. For me I tried both, disbale it and enable it and disable the devices under the option. It didn't work. Still sync from one of the devices to the other - not the other way round.
Or is it synced via Facetime, because some of you wrote, that it stopped happening if you switched of facetime on one or both of the iPhones. So this is not the final solution, but I could help.For me I tried to disable it. It didn't work. Still sync from one of the devices to the other - not the other way round.
Or is it (only) synced via iCloud documents/drive. I thought initially, that this is the work-around, but I need iCloud for both of my work/private iPhones and this is, for what iCloud is designed for.
=> "iPhone Cellular Calls" switched off. Facetime switched off. But still this damned sync. Aaargh.
And some of you have it in both ways from each iPhone to the other. Some of you (as I) have it only in one way from one iPhone to the other, but not in the other direction.
And some of you have it always, but some (as I) only have this from time to time and only by chance. And it starts for some after migrating from one iOS-.x version, for some when migrating to x. version. For me it starts at one point of time, without any update on both iPhones and with 8.x.
This all is really a curiosity, if there is no description, when and with with which iOs-versions and with which settings is should work all the time and in both directions (if really no bug, but feature) and with no descriptions to switch this off, for the users with two personal work/private-iPhones, who do not want this crap.
I think, the "best" way should be the iPhone Cellular Calls-Option. If I want to call with different iPhones on the same number, then perhaps, I want to have the call list synced. If not, not. Or the really best way should be a simple "sync call list"-switch in the Options. Voila.