I agree about the two radios offering redundancy, but having ONE antenna seems to defeat the redundancy argument.
I would suggest that the "noisy" attribute that you mentioned when starting the post is probably due to a poor radio to antenna connection, or bad power (bad filter capacitor / loose / corroded wire). I suggest having someone reputable check over the installation of what you have FIRST. This may save you some headaches later.
I would suggest that you investigate purchasing a handheld unit as your second. Like camradarie, I like what I've learned about the Icom line, and am looking at an IC-M88 (~$250 USD) as a backup. (the West Marine brand of handheld radios are made by Uniden - BTW) This way, if ever the worst case scenario arises, and you have to leave the boat at sea, at least you can take a working radio with you.
Does anyone have any opinions / advice on the IC-M88 radio?