Can a pro boxer compete in bouts sanctioned by multiple organizations, such as the WBA and the WBC?

Yes, but it is virtually impossibe to hold all four belts (WBC, IBF, WBA, WBO) at the same time because of mandatory defenses that the fighter will have to make. The fighter also has to pay sanctioning fees every time they defend each title, which proves that these "alphabet soup" titles are all for themselves and don't care about the fighter. This is the reason why the Ring Magazine came up with their championship policy in 2002 to recognize the true champs in each weight division. True boxing fans such as myself only recognize the fighter with the Ring belt as THE champion, all of the other beltholders are considered "titlists."

Of course, that is was is meant when they say "undisputed heavyweight champion of the world", or whatever other weight class, they hold all of the belts from each organization. If more than one boxer holds a title in a given weight class, they are considered a "disputed" champion since a different governing body has a different champion. Many times, a big fight will be called a "unification" bout, fighters holding different organization's belts will fight with the winner taking both/all belts.

Yes they can have a "unified" sanctioning bout. But it is not always guareenteed that they all their bouts will be sanctioned "unified." a good example is the zab judah baldomir fight. zab was allowed to keep his IBF belt and lose his WBC,WBA titles. the challenger has to be a contender or he is considered a interim fighter to the champion. Now if 2 or mor titles are not online, then normally only one sanctioning body will involved for ranking reason. under most circumstances fighter will usually dropsome beltd to lessen sanctioning fees(3-8% of their purse) to save money

yes, but it`s hard financially cause you have to pay sanctioning fees, case in point carlos baldimir fought zab judah for the wbc ibf belts he beat judah but only got the ibf title why? because he couldnt afford the wbs sanction fee, thats the main reason why you dont see many unsiputed champions nowadays

Oh yeah, just ask Marvin Hagler, Ray Leonard, Michael Spinks, and Roberto Duran, all were undisputed champs (both WBA and WBC).

Yes, indeed.

A fighter can have multiple titles, be ranked one thing in one sanctioning body and another thing in another body.

yes