Michelle, personally if I were going to run them both on 1/2" pipe I would only do so if they were the only 2 items on that run, and I'd begin that run from the 3/4" section.
Newer showers, having water saving heads, don't use near the volume that they used to, and that would be in your favor.
You might determine how well they would work tying into an existing line by checking what else is on the line, and running those items together to see what happened, and decide from there how you feel. There are guidelines on how many items may be placed on different size lines, and I'd certainly recommend following them. I don't happen to have those numbers with me right now, but several people here know them well, and I'd bet that shortly you may have a reply from one of them, and they'll be able to fill in the blanks.