DeaconBlue, the only disadvantage would if you start out by thinking of the viola as a large violin, and attempting to apply violin technique to the viola. I found it easy to switch from violin to viola and back again because I developed a mindset from the outset where I recognized each instrument by feel, and associated treble clef with violin fingering and alto clef with viola fingerings. This is similar to a keyboard player switching back and forth between an acoustic piano and an electronic organ, something else I’m able to do. In both cases, it’s easy to see that some things are the same, some are different.
One famous musician who could make the shift instantly was Israeli violin and violist, Pinchas Zukerman. I once saw a video in which he was playing duets with another famous Isareli Violinist, Itzhak Perlman. They showed Mr. Zukerman first playing a violin duet with Mr. Perlman, and then, right before the camera, picking up the viola and playing a violin/viola duet. The announcer pointed out that making the switch on the spot was a special skill that many string players do not possess.
Actually, DeaconBlue, if you really WANT to play both, you can, and I encourage you to try.