But using tradition in the understanding of Holy Scripture is not to create extra-Biblical doctrines through a selective use of custom, legend, and opinion, calling that "Holy Tradition," and considering it to be a second source of divine revelation equal to that of the Bible.
I think that it is precisely the same thing. They, like the Catholics or Orthodox, see the source of revelation as the Christian community, past and present - that is what constitutes or defines Holy Tradition, and that is what tells "Bible-believers" that they should believe the Bible, and what the Bible includes, and in many cases the rules for reading it correctly.
Since you can't read the Bible in any useful way without this revelation, it is prior to it. If it isn't as reliable as you say the Bible is, you are stuck with saying the Bible can't be more reliable than the revelation of the community about what it is and how to read it. And obviously that information can't itself be from the Bible.
So - there you have a second source of extra-biblical, divine revelation, and indeed one on which Scripture depends for its place and use.
However, my main point is that you are incorrect to say that Protestants disput that they have a Tradition of this type. Some do, but by no means the largest or oldest members of the Protestant groups. Anglicans, Lutherans, and I believe also the Reformed, Methodists, all would quite willingly admit their paradigm or set of rules setting out how to know Christian truth was extra-biblical and the same kind of thing as what the Catholics call Holy Tradition. In fact, their claims to being correct in their views against the Catholic view generally rest on saying their Tradition is correct and the Catholic one is corrupted.
What they would quibble about is what precisly the Tradition says and what it can include, and generally it would be much more limited in what it stated dogmatically that Catholics typically are.
I suspect that the Protestant groups that would deny this actually may represent fewer individuals than the ones that would agree, if not I think that is a recent state of affairs.