Combining various types of groundcovers in a single area can be risky - one is invariably more aggressive than the other and will eventually overtake the weaker selection. Selecting by flower color is also problematic, as bloom times may differ widely. That said, yellow flowering GC's include Potentilla aurea or neumanniana, various Hypericums (St. John's wort), Lysimachia nummularia (creeping jenny), a number of stonecrops and Lamiastrum (yellow archangel). None can be reliably counted on to bloom simultaneously to the ajuga and both the hypericum and lamiastrum are very aggressive spreaders, taller than the ajuga and will easily overwhelm that planting in time. If happy, the ajuga will likely win out over the others in time, as well.

You might do better selecting some taller and more robust type of yellow flowering perennial to compliment the ajuga. In my area, ajuga blooms about the same time as late flowering daffodils - ajuga never forms a dense enough mat to deter bulbs coming through, so that may work also.

You might check out Green and Gold (Chrysogonum virginianum). Maybe it will match your needs.