But trustees of the two organisations found they had a great deal in common, and this week the two organisations announced they would merge.
"It seemed a natural fit," said Joyce Moseley, who has been chief executive of Rainer for eight years and will head the new charity. "The more we talked, the more it made sense."
She stressed that unlike other mergers, this one was not driven by finance, but by a simple desire to do the best for beneficiaries.
The two organisations, she said, had simply grown together: "Quite often, when charities merge, it's because one of them is struggling.
"But that's not the case here. We have similar funding streams, largely from government contracts - though Rainer has access to funding from education sources, unlike Crime Concern.
"We're both in good financial shape, but this just seems to make perfect sense. We can achieve more together than we can alone."
Clare Checksfield, chief executive of Crime Concern, said: "The reasons for the merger are long-term and strategic. It gives us greater geographical reach, access to more young people and a better understanding of the problems everyone faces. It allows us to offer more services and to be more flexible.
"We'll have a stronger presence when we compete for funding, and a more powerful campaigning voice. It makes a lot of sense."
The merged charity will have an income similar to that of the National Gallery and will come in at 119 in the Charities Direct list of the top 500 charities by income. At present, Rainer is ranked 262, Crime Concern 301.
The two charities, Moseley said, had grown together over the past 20 years, despite differences in their histories and founding objectives.
Crime Concern was originally created to ensure safer, crime-free neighbourhoods, Rainer to offer opportunities for young people in deprived circumstances.
But Crime Concern has increasingly found it can cut crime by helping young people not to offend in the first place.
And it claims this prevention-is-the-best-cure strategy has helped cut crime in some neighbourhoods by about two thirds.
Both organisations now focus their efforts on young people who are likely to fall into lives of crime. The two charities also have similar approaches. Both of them focus on volunteer mentoring and try to encourage community involvement, although Rainer also provides housing for homeless young people.
The new charity will work principally on solving young people's problems in four areas: housing, school, family and community.
"Even a cursory glance at the two websites reveals how much common ground we have," said Checksfield. "We do very similar things."
Objectives: The supervision, maintenance, education and employment of young people, especially those in danger of becoming young offenders
Activities: Providing one-on-one mentoring, sheltered accommodation and training; working to create opportunities for young people
Objectives: Protecting people and property from crime; providing activities for young people at risk of offending; researching new techniques in local crime prevention
Activities: Providing one-on-one mentoring for young people at risk of offending; creating opportunities for young people.