I'm a long time solo player, but have just started playing solo with 2 decks and have what is probably a pretty basic rules question. Near the end of the rules it describes how you can play attachments on other player's cards, hence giving them control. I know there are some ally cards that can also be transferred from player to player. My question is with respect to what happens to the cards when they go out of play - I believe the rules are pretty clear in that they state that they always go back to the original owner's deck/hand/discard pile, not the player who currently has control. I just want to make sure I am interpreting correctly because this means you need to keep track of who owns what, which will be pretty difficult to remember correctly unless everyone use different color/type of sleeves for their decks, or everyone is playing completely different spheres with no overlap. Since I'm playing 2-handed solo I'm planning to use 2 different colored sleeves - any other suggestions?

Yes, it can get difficult!

And in answer to your question, yes you're correct. Discarded player cards always go to the owner's discard pile, regardless of who was in control of them at the time of discarding.