Yes you can as long as both paints are water based, latex or acrylic. The biggest problem you could run into in the "non-standard" sheen you will get. This will wind up similar to a satin but not quite.
Every manufacture has a different definition of specific sheens. So if you wind up not having enough, for a second coat or touch-up, you will be somewhat screwed.
Also, as mentioned above, you will have to mix the 2 gallons very well or wind up with a funny looking mess. Make sure to use a 5 gallon bucket and a paint mixer mounted to a drill. Mix for several minutes before painting.
Keep an eye on the mix. Sometimes different paints from different manufactures won't play well together. You will know if this is the case.