I think it may be a christian concept tal....
The first commandment is first found in Deuteronomy 6
5 Love the Lord your God with all your heart and with all your soul and with all your strength.
The second commandment is found at Leviticus 19
18 Take no revenge and cherish no grudge against your own people. You shall love your neighbor as yourself. I am the L ORD
However, if you read the a bit further in Deuteronomy you start to see that Loving you lord with all your heart refers to not worshipping idols, and treating each other kindly only ensures that all will go well with you...they are seperate issues.
Deuteronomy 13 Fear the Lord your God, serve him only and take your oaths in his name. 14 Do not follow other gods, the gods of the peoples around you; 15 for the Lord your God, who is among you, is a jealous God and his anger will burn against you, and he will destroy you from the face of the land. 16 Do not test the Lord your God as you did at Massah. 17 Be sure to keep the commands of the Lord your God and the stipulations and decrees he has given you. 18 Do what is right and good in the Lord ’s sight, so that it may go well with you and you may go in and take over the good land that the Lord promised on oath to your forefathers, 19 thrusting out all your enemies before you, as the Lord said.
I think that the understanding of the second commandment was changed by Jesus when he referred to whatever you do to them you do to me. Therefore giving a christian idea that doing this is the same as loving god with all your heart.
However, he did not say they were new commandments, he said they were the greatest. So maybe it wasn't him that tried to say they mean the same thing but us?
Deuteronomy 6:20-24 then explains the meaning of these commandments...
20 In the future, when your son asks you, “What is the meaning of the stipulations, decrees and laws the Lord our God has commanded you?” 21 tell him: “We were slaves of Pharaoh in Egypt, but the Lord brought us out of Egypt with a mighty hand. 22 Before our eyes the Lord sent miraculous signs and wonders—great and terrible—upon Egypt and Pharaoh and his whole household. 23 But he brought us out from there to bring us in and give us the land that he promised on oath to our forefathers. 24 The Lord commanded us to obey all these decrees and to fear the Lord our God, so that we might always prosper and be kept alive, as is the case today. 25 And if we are careful to obey all this law before the Lord our God, as he has commanded us, that will be our righteousness.”