Well, this only happened this particular run through, after 3 other run throughs of being forced to choose. I spent a lot of time early in the game talking to all the companions and getting their approvals high as soon as possible, so I think I triggered Leliana's quest between Redcliffe and the Urn, and hardened her long before she reached the love stage. That may have had something to do with it.
Morrigan doesn't want to share either, btw, none of them do normally, so she made me choose. It's Leliana who didn't care that I chose Morrigan, she continued to be in love with me, and I still had all the love dialog options available. Of course, I also never chose to break up her either. Morrigan was apparently satisfied that I had chosen her, and was blind to all the times I continued to invite Leliana to my tent. Morrigan also gave me her ring twice, which is why I think this particular run through may have been bugged.
Okay, here's the key to Leliana. There is one particular conversation that makes all the difference. After completing her sidequest, you have the conversation where you can harden her or not. After that, there's another one where you talk to her about Marjolaine again. There are 3 particular dialogue options that are critical:
"You miss her...."
"She meant a lot to you...."
"Someday I hope to mean as much..."
If you choose those dialogue options, it will rekindle Leliana's romance even if you dumped her before. So if you chose Morrigan when Leliana asked you to choose, then had the above conversation with her afterwards, then you're golden. You've got both girls. Morrigan will tell you go break up with Leliana again, but like I said in a previous post there will be no option to actually do so, and then neither one of them mentions it again.
It works the other way as well, but is more simple. When Morrigan asks you choose one or the other, you can choose Leliana. Then go to Orzammar and buy the golden mirror. Give it to Morrigan and tell her it's "just a gift for a beautful woman." Viola! You've got both girls again.
I usually choose Morrigan first and have that critical conversation with Leliana later. Why? Because I want to wait until I've got my coercion up to at least 3 before doing Leliana's side quest. Why again? Isobella. Nuff said.