Based on a 2.1-ct eternity band of 10-pt melees (21x0.10), I am guessing that the melees of this 2.5-ct Garland ring are around 5-pt for smaller melees and 10-pt for larger melees. The diameter of a 5-pt melee is about 2.4mm. Two of the 5-pt melees stacked together will have a width of about 5mm for the ring. The diameter of a 0.5 ct is about 5.2mm. Therefore, I imagine the size of the Garland eternity band be quite substantial. It is like wearing an eternity ring made of 0.5-ct diamonds. Therefore, I think you can wear the Garland ring alone. Anything added to it will be too much and may detract the beauty of this eternity band.
The diamonds in my WB are over 0.4 carats, and I am wearing mine by itself. No additinal bands for me. If I do, I would be very careful about the height and diamond melees of the stacked bands and make sure that the diamonds from both stacked bands are not damaging adjacent diamonds and metal.