I see what you're saying. Going by US tax law, somebody running their business as a full corporation could play these accounting tricks at the company level and repay the loss later with future income -- just make sure you have a plausible story in case of a tax audit. Any other structure, both income and losses automatically pass up to the business owners and hence the taxable reported income will not change. (Unless you report phantom income and pay taxes on income you didn't make.)
Well, if you're both sole proprietors, why would that make a difference? Perhaps Canadian tax law is entirely different, but Jane would experience a loss in business income, which would reduce her taxes. Jack would see an increase in income. Oh, I see, the net income for Jane would be less. But Jack and Jane could go to their bank, show Jane's pay cheque stubs and prove her $100k income, while Jack could produce his year-end tax statement to show his income. So, to the bank, the total income would be $150k.
(To be clear, I'm not talking about phantom income. Jack is seriously doing work for Jane and getting paid. Jane seriously hopes to make money off the work Jack does, but down the road.)