Linking two client sites together - seo query?

I've got an offline client who is breaking into an area that requires every seo element possible. Solar Panels - so it is for local trade

I've purchased a keyword url and have been buidling a site

but

he also wants to add other areas not related to that keyword - local plumbing and heating (which I've just found out about today after half building the site under the solarpanel keyword url)

I have a brand name url and could build a site using that

heres my question

If I build both sites with the same theme and layout and use the canonical element to ensure that google only reads one page on one of the sites and I link the two sites together by adding url links to nav and sidebar menus that take people to both sites

Is that an okay thing to do? I don't want to mess up their seo before we even get started?

so both sites will have links to each other and I will use canonical? And using the canonical is new to me - do I just add the url of the site I want to be indexed into the page that I don't want to be indexed.

I plan to have one site for solar panels, the other will host all information about their plumbing and heating and I will link them using 'menu'

What do you think - ok seo or bad seo?