one thing you can do, is take some aluminum foil and cut it to fit your mold, then when you inject you get 2 halves, you will have to fit them back in the mold to inject the 2nd color, which can be a pain, but it works pretty well. this is what i did until i bought a twinjector.
i bought the basstackle twinjector (6oz version) and am very pleased with it, very high quality, locking pins on the injectors, very high quality unit. it does have a bit of a learning curve to it, but its not too bad, took me about 2 times working with it to get it down. (though i still havent mastered the ripper!)