No one has an opinion?
That seems a valid approach to me also. Make an agreement for keywords and not niche but in case you get two tough competitors in same niche and almost same keywords, leave the 2nd one and stick to 1st client only.I've had this issue come up before and I decided that SEO isn't based on niches, it's based on keywords. Given how important it is to be #1, I wouldn't think it very ethical to deal with 2 companies for the same keyword, but two companies in the same niche with different keywords is perfectly fine and non-competitive.