Note: the Collateral Factor of the lent asset(s) is multiplied by the Borrow Factor of the borrowed asset(s) to arrive at the final factor.
For eg if you lend 1000 USD worth of USDC, you can borrow UNI in line with a final factor of 0.648 (0.90 x 0.72). Hence, 648 USD worth of UNI.
Alternatively, if you lend 500 USD worth of USDC and 500 USD worth of WETH, your risk-adjusted collateral value is (500 x 0.90) + (500 x 0.88) = 890 USD. If you were to borrow UNI, you could borrow 890 x 0.72 = 640.8 USD worth of UNI.
Note that if you borrowed less UNI, for eg 500 USD worth, you could still borrow additional UNI or a cross tier asset like LINK against your risk adjusted collateral before hitting the threshold.