A wallet will mine when it has mature coins and at least 1 transaction and at least connected to 4 other nodes. However you want to get transactions into a wallet is up the user. There is a Python script in the white paper, the community has made a Nodejs version that sends txs, another dev is working on another approach. You can have many wallets with the GUI, but only one of them can be active at any given time. This means the miner will mine using the active selected wallet. Easiest approach is to use 2 GUI wallets. Send txs from one wallet to the receiving wallet.
There is much mining talk on Telegram, come join the chat and ask the community questions.
https://t.me/BitcoinPoWPoTThat's interesting, because I'm just testing this just now, and the "inactive" wallet found a block. Maybe just a gui bug?