By buds I assume you mean the tiny immature fruits at the base of the blossoms. Perhaps one tree is more mature. Are they the same variety of feijoa or are one or both seedings? If they are different varieties, did they bloom at the same time? Make sure they are getting equal water--it can be very hard to evenly moisten soil in a pot.

It is normal for some early stage fruits to fall off, but you would expect both plants to behave similarly.

Correct, look like tiny fruits. I purchased both from the same farm, very similarly size as well so i assume they're the same variety. They're about 7ft tall so quite mature I think. Both bloom at the same time.

I have them on drippers with the same exact head on them. The taller of the two is actually the one not fruiting well.

If they were not sold as a named variety, and nurseries often charge a premium for this, they might be seedlings -- unless that farm does not sell seedlings.

Otherwise too soon to tell, could just be natural variation of two plants in the same year. Try running your fingers or a small brush through all the blooms, moving pollen around next year, this is commonly done to feijoa.