it would be very nice of you to get the other woman something, whether she's a flatmate or a relative of your lady. you can just get a small bouquet or a single flower for the other woman, make sure your ladys flowers are bigger and more special though, cos if you get two exact same bunches of flowers your plan of showing her how much she means to you will not work out! :)
....and to be on the safe side take one simple flower for the other woman...:)
These ladies know!