I'm wanting to start a herb garden this year, and I've found Jekka McVicar's website a good starting point
http://www.jekkasherbfarm.com/index.asp
She also has a great book, which is on my Christmas list!
I shouldn't think you'd have much trouble filling the plot, there is a massive range of herbal/medicinal plants, and some of them are huge!! (Thinking of comfrey & lovage for instance). How much extra trouble would it be having to travel to 2 different sites to tend to your plots though? Even if your original plot is 'fettled' already it will still need a reasonable amount of time in the growing season, and the herb one sounds like it needs a fair bit of time to get it ready and will also need maintenance once established. If you're sure you can cope with both, and the travelling between sites, then I'd say go for it!