Get yourself some dust from a block mined on one of the chains.
Do you think it would be better to use fiat to buy a small amount on an exchange, or is it better to have a miner running that can mine some dust in a more reliable and timely manner? If so, which miner would you recommend (e.g. 21co)?
Combine that dust with your existing coins by sending a transaction to another address that you control. Since those coin base rewards don't exist on the other chain, this transaction will only be valid on one chain.
Let's imagine I have a cold wallet with a stash of 3 XBT. I assume I can import these private keys into *both* BU and Core wallets at the same time. On each client, I should see a balance of 3 XBT.
Now I start to work with the BU CLI: I need to craft a raw transaction with 2 or more inputs, and 2 or more outputs that go back to my own BU wallet. Is that right?
After that, send your coins on the other chain to a new address that you control (extra measure just to be sure). At this point, your coins will be effectively disassociated between the two existing chains.
So now I open up my Core wallet, get a new receiving address, and then use my BU wallet to send my entire BU balance to my Core wallet. After no less than 1 confirmation, I see my Core balance is now 6 XBT, and my BU wallet is 0 XBT. Correct?