I have been on a couple of offshore boats that have more than one VHF radios. I am going to be doing some offshore trips this year. Should I invest in another radio.
Yes absolutely; it's not really even acceptable to only have one radio even on an inshore boat, between the radio's, antenna's, and wiring VHF's are about the least reliable electronics we have on our boats and they often fail or work poorly.
Keep in mind though once you get about 15 miles away from land or the nearest boat VHF is useless and you need to employ a different technology.
If you run into questions about what type of VHF, it pretty much comes down to price range and your personal taste.
I'd reccomend pretty much anything permanent mount (ie 25 watt vs. 5 watt handhelds) that does DSG. On my boats I buy Uniden (they are also sold as "West Marine"), they work, are reliable, and have the features I want (ie submersible rated, programmable scan, with DSG and a manual squelch) for a good price - that model is about $150..
As far as antenna's, I'd reccomend buying the best you can in the length you want with a good quality stainless mount.. under $150 for an 8 ft and the mounts are under $50.. however with that said sometimes my cheaper antenna works better than the expensive one, which is pretty much the bottom line 8 ft. glass whip that you can get in a metal base..
Most of the time though the expensive antenna does work better and for the heavier guage wire it's equipped with alone, it's worth the added investment.