I put a large spousal loan in place this year. Do i still prepare my spouse's return join with mine or do i need to prepare them separately?

Both returns are separate in Canada, anyway.  There are no joint returns, they are just connected for tax purposes.  Is this a business related loan, or is this a spousal RRSP contribution?

For the spousal loan, is it at the current rate of interest, and are you claiming the interest paid on your own tax return as investment income?  If the rate of interest is less than the going rate, then you run the risk of being subject to the rules of attribution.  This states that unless fair market value was paid for the transferred assets, then any income from said assets are attributed back to the original holder.  Please see the following link for more details.

http://www.cra-arc.gc.ca/E/pub/tp/it511r/it511r-e.html

But WHERE does one report the spousal interest( paid back to the loaner!)?

Did you ever find the answer the this question? Is it "Other interest income not reported on a T5?" So reported as a line item as interest in investment income?

If you are using TurboTax Online, please do the following:

1. Use the Find tool 🔍 and enter “invest” in the search bar
2. Select “Investments” from the search results, then click the Go button.
3. Check off “Interest and Other Investment Income” and click Continue until you get to the “Interest and Other Investment Income - Summary ”. There you can enter your Interest income. 

But as TurboTaxJeffrey said, make sure that you are aware of the rules of attribution, and that your loan doesn't run afoul of them: https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/it511r/archived-interspousal-certain-other-transfers-loans-property.html