Here's another pic of the amp:



I think that 60W would be a little small for 2 vocals and a guitar. Maybe you could work it though, since you don't seem to need huge volumes, but I just think that it'll be hard for one speaker to handle 3 inputs. I used to use a 150W (I think) keyboard amplifier for 2 vocals for band practices, and it would freak out when I plugged in a bass (don't worry Doc, there were dedicated instrument and mic inputs on each channel). The amp sounded best when there was only one mic or one keyboard plugged into it, even though there were 4 channels.

What kind of practice amp do you have for your guitar? Maybe the best setup would be to run vocals through the Samick and your guitar through your guitar amp.