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No they should remain separate installations and both are independent from each other!
thank you Alfred. that's what i was thinking, but i've been surprised before. do you know that as a fact? personal experience or someone who has done it? once i do it, going back could be impossible. thank you again.
If, as you say, they are totally separate installations on separate hard drives, each with there own version of PS for example, there should be no way they could interfere with each other! I presume you are using some form of boot manager to load a specific version of OSX?
I have done a similar thing a while back in a similar situation with client compatibility, and didn't have a problem with either interfering with each other!
On the other hand, you are sharing the programs somehow between the different OS's then it would be a different matter!
thanks Alfred, you've been a big help. i'll boot up in el capitan and try the upgrade in the next few days.