Elements bundle, photoshop and premier 15 and 18 can I have them both.

I just went through a slight bit of agony because I was running my iMac off of an external hard drive. Once I realized what was happening and after a few tense moments things started to fall into place. I was having a problem downloading Photoshop and premier elements. Once things were straightened out things became quite a bit clearer. Morral: downloads don't work when running of an external drive. Now that it's straightened out, I only have one question. If I keep the Elements 15 canI install the Elements 18 and use either one or are there going to disappointing conflicts form having the two on my iMac?