I will prefer you to chose ETH first and secondly LTC. Because ETH demand is high than LTC. You can split your investment like more than 60% in ETH and rest of percentage for LTC.
If there are only two coins in the portfolio, then 50:50 or 75:25 (75 for ETH) can be an ideal ratio. Litecoin is less volatile when compared to the other cryptos. But it's future potential is limited, unlike that of ETH.