I have found it quite difficult to find any commentaries, ancient or modern, that state that the "us" is not Jesus and John the Baptist. Your question however has challenged me to look outside my orthodoxy, and so I present two interpretations:
1. Jesus was referring to himself and John the Baptist
First, Jesus himself had to be baptised, and he was aware of this. Ignatius of Antioch in his Epistle to the Smyrnaeans writes that Jesus was:
Baptised by John for His fulfilling of all righteousness;
Thus the church since apostolic times has interpreted that Jesus was referring to himself and John when he said "us".
Secondly John the Baptist, although in his humility saw himself as unworthy to baptise Jesus, saw his part in Jesus' fulfilling all righteousness. This is evident from John 1:31 (ESV):
I myself did not know him, but for this purpose I came baptizing with water, that he might be revealed to Israel.
John baptised Jesus, after Jesus accepted John's humility, but not his refusal. Here we have an image of not only Jesus' submission to His Father's will, but John's submission to the will of the Son. In doing so, together they fulfilled all righteousness, anointing the Christ for His ministry.
A dispensational view emphasises the importance of Jesus and John being the "us". The Companion Bible in its note on "thus it becometh us" in verse 15 states:
This duty was incumbent on John as a minister of that Dispensation; likewise on the Lord: hence the word "thus".
2. Jesus was referring to himself, Mary, and John the Baptist
There were, of course, more people than John the Baptist and Jesus present at the baptism of Jesus, presumably the apostle John or the witness who relayed the event to him, and probably some of John's disciples. According to the Gospel of the Nazarenes, Jesus' mother and brothers were also present, and indeed took part in this event:
Behold, the mother of the Lord and his brethren said to him: John the Baptist baptizes unto the remission of sins, let us go and be baptized by him.
The Roman Catholic Church holds the view that Mary was critical to Jesus performing his ministry on earth. A key objective of Christ's ministry on earth was fulfilling the promise of Genesis 3:15 that the serpent [Satan] will be crushed. Pope Pius IX in Ineffabilis Deus writes:
...the most holy Virgin, united with Him by a most intimate and indissoluble bond, was with Him and through Him, eternally at enmity with the evil serpent, and most completely triumped over him, and thus crushed his head with her immaculate foot.
To fulfil all righteousness, Jesus not only required John the Baptist to suffer him, but Mary's intimate bond with him to "fulfil all righteousness".
Summary
It is clear that Jesus and John are in the "us", and this is historically the orthodox interpretation of this text. If however, look beyond the orthodox, we can add to this pair Mary. This however is not my personal view, but this could be maintained from a Roman Catholic theology. The first more Protestant of explanations I found much easier to research into and agree with.
As a final note, I find it interesting that πληρῶσαιἡμῖν is plural, not dual. That said, my understanding is that dual is rare in late Greek, except δύο.
