Here's an idea and you get the same effect so to speak. Have two glass cylinders with different colors of sand in each. When it is time during the ceremony, bride and groom each take a cylinder and pour sand together into a larger cylinder. It will still signify the two becoming one!! It looks so cool.
Whatcha think?
Blessings,
Winnie
PS: I make unity candles with the wedding invitation on it. I just made one today for our vow renewal on July 22nd. I'll take a picture and post it as soon as I can.