Answer:
The first loan for $8,000 could fall under the exemption of employer-employee loan. But then after the second is taken, that exemption would no longer apply. A minimum interest of $18,000 x 4% x 6/12 = $360 should be charged.
If the loan is considered a corporation-shareholder loan, then it doesn't qualify for any type of exemption, resulting in interests = ($8,000 x 4% x 6/12) = $160 for 2020
for 2021, interest applied = [($8,000 + $160) x 4%] + ($10,000 x 4% x 6/12) = $326.40 + $360 = $686.40