For Linkearning you need content wich is readable/shareable/linkable - really great stuff with a lot of value for others, without great content it is a Mission Impossible. So if thats part of your "onpage" - you need great value to share with the right people.
It's hard to answer this question, we don't really know how far u are and what you exactly mean with onpage or what the site's content is.
I think Links to products are pretty hard to get. You need content round about your products, linking to them. Outreach for Links to products could make you sad and depressive ...
PS: I think Rands last WBF helps a lot in saving time by do outreach in a better way: https://moz.com/blog/link-building-outreach-in-a-skeptical-world-whiteboard-friday



A new site: I would try that way (but that is my liking). I would Link both sites to each other where ever (natural and relevant) possible. The way for the B2C site is long, in both ways B or C, but in case C, you can link back to the B2B site, so the old site can benefit. There are so many opportunities. I think it should be also possible to link them sitewide, I guess Google wouldn't see that as a manipulative linkbuilding. To much fear? Ask John Mueller in a webmaster hangout for his best practice tipps.