Shah, former minister of state for transport, had left BJP ahead of 2012 Assembly elections and fought as independent from his home turf, Kapadvanj.
However, he lost and stayed away from politics for a while, without joining any party.
Lakhani, a prominent lawyer and the son of former Gujarat Assembly speaker Shashikant Lakhani, too joined BJP with hundreds of his supporters.
State BJP president R C Faldu welcomed them into the party fold at the function, in the presence of cabinet ministers Babu Bokhiria and Bhupendrasinh Chudasama.
Defending his return, Shah said he had quit out of anger.
"I took that decision with a heavy heart. I never joined any other party after I left," he said.
Lakhani claimed he did not set any conditions for joining such as election ticket. "I was with Congress for 18 years. I am not leaving that party due to any injustice to me, but I am joining BJP to realise Modi's vision of growth," he said.