If anyone-can-mine=true, default address (the first address of the node) is always used for mining. So, you will see only one address from the node.
As for the question why only one node mines blocks. The answer depends on the target-adjust-freq setting.
If it is -1 (default, no POW) - there is clear preference for the "old" miner (mining-turnover). If you will stop the first node - you will see blocks from the second node very soon,
If it is not -1 (POW, I suppose it is not a case) - probably your second node is weaker than the first.