My sister had the same situation, with me and one of her other best friends. She ended up choosing both of us, and we split the "maid of honor" duties. One of us toasted at the rehearsal and dinner, and the other made a toast at the reception. One of us held the flowers during her vows while the other made sure her dress was straightened.

If you want to choose both of us, it is definitly ok. I would make a job list for both of them, and tell them that they are sharing the honor because you love both of them equally and want them both to share that with you. After that, tell them that they can divide up the maid of honor duties between themselves. Whatever they feel comfortable with is what they should do. I ended up doing the toast at the rehearsal and dinner because I am not comfortable with public speaking and I knew that the guest list at the rehearsal would be limited to wedding party and spouses only.

I say, if you choose both of them, let them decide which of the duties they want to do based on what they are comfortable doing!
Good luck on your decision, and remember that is your day, and you do whatever makes you happiest!