gumpaste & fondant together? I mean, if you need to use GP then why not just use it alone..What difference does the use of fondant make except that it takes longer to dry?

A more technical answer may come along from someone decorating longer than myself, but when I make scultped figures and I need a stronger hold, but still need time to 'work and play' with my modeling, I mix GP and fondant together.... the GP ultimately gives me a stronger hold while the fondant keeps the mixture softer a tad longer for me to work with. HTH!
~ Jaime

ok what a bold post I think we got your message loud and clear. And I mix it because fondant most of the time is cheaper than gumpaste. If you mix it, it helps spread the gumpaste and you can make more decorations. Especially since I make marshmellow fondant. Also its easier to work with and I can take a little more time before it sets. Hope that makes sense.

What Jaime said..

When you are doing figure modeling, straight fondant doesn't set up fast enough and your larger structures can sag. Straight gumpaste sets up nice, but too quickly for details. It gets a skin and wrinkles up on you when you bend it. Mixing them gives you the best of both worlds.

I mix them for the reasons mentioned by the previous posts,and also because I use precoloured fondant...so it saves me having to colour the gumpaste,or having to try to match it...it's already the same colour that way..if that makes sense