I am learning about radicals and their use in forming an individual Kanji character, but I am wondering if more than one radical can be used in developing one. Thank you.
Strictly speaking, each kanji belongs to only one radical. According to Wikipedia (emphasis mine):
A Chinese radical (Chinese: 部首; pinyin: bùshǒu; literally: "section header") is a graphical component of a Chinese character under which the character is traditionally listed in a Chinese dictionary. This component is often a semantic indicator (that is, an indicator of the meaning of the character), though in some cases the original semantic connection has become obscure, owing to changes in character meaning over time. In other cases, the radical may be a phonetic component or even an artificially extracted portion of the character.
For example, the radical of 男 is 田, not 力. The radical of 音 is 音, not 立 nor 日. The radical of 嬲 is 女, not 男 nor 田 nor 力. Confused? Don't worry, I'm also confused. And that's why, in higher levels of kanji-kentei, there are questions which ask you to point out the correct radical of a kanji.
However, ordinary people may use 部首 somewhat loosely, and it may refer to any recognizable component of a kanji. Modern IMEs have a 部首検索 ("search by radical") function, and you can usually use whichever component you may recognize.
Knowing 部首 is definitely worthwhile to a certain degree, but don't be too much bothered by difficult and tricky ones :)
Yes. When there is more than one radical, you first need to identify the primary radical. The convention is for the primary radical to be determined by its placement: (1) top, (2) left, (3) bottom, (4) right.
"Grass" (kangxi #140) on top. "Tree" (kangxi #75) on bottom. So, two radicals, but you use the grass radical to look it up in a dictionary.
"Female" (kangxi #38) on left. "Child" (kangxi #39) on right. So, "female" is reference radical, but there still are two radicals.
"Grass" (kangxi #140) radical on top. "Water" (kangxi #85) radical to left. Kangxi #41 ("thumb / inch") radical on bottom. So, the top radical is most important. The reference radical is "grass".
Organizing kanji is already difficult enough. Making a kanji such as 薄 referenceable by all 3 radicals would just be untenable. The (top / left / bottom / right) convention solves that problem. Of course, there are exceptions to the convention such as kangxi #163. Don't ever give-up until you look under kangxi #1.
Also, the number of strokes in a radical enhances its precedence... yes. it can be very frustrating using kanji dictionaries such as Nelson. People these days just use handwriting recognition. For example, one dark day I was brought to tears looking for "音". I kept referencing by the smaller radicals I saw. But in fact the whole kanji is a radical by itself. You've got my respect for using dictionaries instead of handwriting recognition!
I just came across a perfect example of how radicals can drive you crazy:
The more complex radical (9 strokes) that is in the top position, 音, is somehow not the reference radical. Rather, it is the simpler radical on bottom, 心. You gotta love it.
There are multiple types of radicals. The traditional radical is the or Japanese bushu, Chinese Kangxi, both written in kanji as 部首.
With the advent of the computer a different kind of Kanji dictionary came into existence. I think this multi-radical method was developed by Jim Rose of KanjiCafe. This dictionary allows you to select all the "radical" sub-kanji that compose the Kanji.
My old Canon Word-tank denshi jiso uses the bushu radical lookup method.
Most of the online western kanji dictionaries use multi-radical dictionary lookup. For example, see the dictionary on Jim Breen's site or Denshi Jiso. I've been using Jim Breen's site for years and I think it was the first on-line multi-radical dictionary. The radical look-up files it uses and distributes are credited to Jim Rose.