recording vocals

I need to create a musical accompaniment track, playing a Sibelius musical piece that I wrote, and recording my voice x 2 over it, to make a single piece with instruments and 2 voices, that another singer and I can perform to. What is the best way to play the track and simultaneously record the two vocal parts? I'm thinking playing it on iTunes, and recording the vocals twice on WireTap Studio using an external microphone hooked up to the Mac mike jack. Wire Tap is installed on my Mac (OS 10.7.4). Afraid I might get some loss of quality, even tho WireTap claims to be "lossless." Does anyone know of a better way?

Tom in Texas