Wheat
Wheat carries the same powdery mildew, smuts, and aphid-spread viruses as barley, so growing them side by side stacks the risk and gives spores a short hop between stands. Keep small grains apart and rotate the bed away from grasses for 2 years.
Oats
Oats host many of the same leaf diseases and the aphids that spread grain viruses. Planted next to barley they extend the green bridge that carries those problems from one small grain to the next.
Rye
Rye shares barley's disease list and is a vigorous grass that also out-competes it for water and light in a mixed stand. Give each small grain its own bed and its own year.
Oregon Grape
Barley is hit by the same stem rust that overwinters on barberry-family shrubs, so a nearby Oregon grape gives the fungus the bridge it needs between seasons. Plant grain plots well clear of the shrub.
Emmer
Both are grasses that carry rusts, powdery mildew, and grain aphids, so planting them side by side lets problems bounce between the two stands. Grow one grain per bed and rotate out of grasses for two years.
Einkorn
Barley and einkorn share rusts, powdery mildew, and the aphids that carry barley yellow dwarf. Growing them side by side gives spores and aphids a short hop between stands and stacks the risk on both.
Spelt
Barley hosts the same rusts, powdery mildew, and aphids that trouble spelt, so side-by-side rows let problems build in both. Grow other grains in a separate bed and respect the two-year gap between grass-family crops.