I have romanced both characters, but I didn't realise this would impact the storyline and I have since decided that I wanted the so called 'Yennifer ending'. Is there a way to still get the 'Yennifer ending', even though I have romanced both women?
No, the ending is based off of your choices you make throughout the game. Since you have made your choice, the ending will reflect what you have done obviously. You have to avoid romancing Triss throughout the game to get the Yennifer ending.
MASSIVE SPOILERS HERE (SOURCE): http://www.ign.com/wikis/the-witcher-3-wild-hunt/Endings