I've performed OE-PCR to amplify two amplicons (2.8 kb and 1 kb) as a single fragment. I first perform 10 cycles of PCR without the primers to allow fragments with overlap regions of about 30-40 bp, act as template for each other. Then post 10 cycles add primers and continue next 25 cycles. But there is no amplification of expected size (~3.8kb). Could anyone please provide feedback on troubleshooting the same.

Thank you in advance.

P.s. I've included the gel image for reference.