These are the two places where Cantonese is one of the official languages, so you can expect a majority of the population to speak close-to-native Cantonese. Mandarin is also on the rise here given the influence of Mainland China and especially its importance for doing business. Having said that, not everybody in Hong Kong and Macau can speak Mandarin, a good guess (based on living there) would be that around 70% can speak good Mandarin, especially older people and people living further away from the central districts (in Hong Kong) often have trouble communicating in this language. Nevertheless, a majority (more than 50%) of people in the two regions should be able to speak both, Mandarin and Cantonese at a good to very good level.
In major parts of Guangdong and the eastern part of Guangxi Cantonese is the language people used to grow up with, so most of the native people do speak both, Cantonese and Mandarin. Having said that, the influx of people from all over the country has led to a situation, where many people for example in Shenzhen don't speak Cantonese. Most of them are able to understand at least a bit, but can't really speak. In Guangzhou, the situation is a bit different as people are still very fond of the Cantonese language and to really fit in there, learning Cantonese is important, but in places like Zhuhai or Shenzhen, Mandarin has taken over many parts of daily life. Another trend in those areas is that many kids of Cantonese speaking parents now grow up actually speaking Mandarin as this is the language used in kindergarten and primary school. They still understand Cantonese, but don't really speak it.
In many places all over China there are people who grew up in a Cantonese speaking household in the south and now live in a Mandarin (or other dialect) speaking area somewhere else in China. Those people speak both languages perfectly out of necessity. Furthermore, Cantonese songs are hugely popular all over China (and beyond), so many Mandarin speaking people also learn basic Cantonese just by singing and listening to Cantonese songs.
Both Singapore and Malaysia have a big Chinese community and many of the people there can speak both, Cantonese and Mandarin. In fact, even many other people e.g. in Singapore can make basic communication in both languages just from growing up in areas where both languages are used on a daily basis.
Cantonese people used to be the majority of Chinese immigrants decades ago, so when going to Chinatown in any major city around the world, it is not unusual to hear more Cantonese than Mandarin. This together with the importance of Mandarin today leads to the fact that many foreign born Chinese who grew up in a Cantonese speaking household can speak both Cantonese and Mandarin having learned Mandarin either in school or later on for business.
As mentioned in the introduction, it's difficult to come up with exact numbers for how many people can speak both Mandarin and Cantonese. Depending on the requirements, the number can be relatively small (both native) or pretty big (basic communication). The surprising thing might be to see that those people can not only be found in the southern parts of China but in many places in Asia and around the world.