I am running low on my Satin Ice fondant but have a freshly made batch of MFs fondant.
Do you think it would be safe to combine the two to cover my cake?

I don't see why not - I have mixed MFF with gumpaste which has a different consistency, so you should be able to mix it with another type of fondant - just be sure to mix it together thoroughly.

Sure. Knead them together well and if you have the time, wrap them up and let them rest once kneaded. You'll probably love the results--the best of both worlds!

Rae

I've done it when I wanted to marble two colors together and had one color of each available. Worked great for me!

Thanks everyone! I would rather be on the safe side and know that I have enough to cover it so I am going to combine them and let it rest over night.
I am a nervous wreck over this cake for some reason.

I always mix my Satin Ice with another fondant (white)as the colors are just to saturated and by adding another fondant it also helps to lessen the stickiness of SI. I mix Pettinice with SI - not a ton - just enough make it handle better and not enough to change the color any.