No, this is not possible. Every block in the chain has a reference to its parent block (except the Genesis block). So If you could merge two chains then you would have to change the parent block of one of the blocks in the chains. So, no, this is not possible.
Think of it like that:
[chain_0] Genesis -> ch0_block1 -> ch0_block2 -> ....
[chain_1] Genesis -> ch1_block1 -> ch1_block2 -> ...
In order to merge the two chains you would have to do something like the following:
...ch0_block2 -> ch1_block1
This is not possible though since each block carries a hash code which has reference to its previous block.