The headteachers of the two schools already work closely together and it is clear that their success in becoming joint schools of ambition will cement the relationship further.
Plans include finding ways to enhance the curriculum, particularly at the upper stages of the school. Both have rolls of only around 600 pupils, due to the demographics of Glasgow's outlying housing estates.
But by co-operating and sharing timetabling, the schools feel they can enhance what they are able to offer.
Brian McAlinden, head of Castlemilk High, said: "We want to get various curricular structures in place so that pupils can move from school to school and school to college." The plan is that Castlemilk High could offer physics at Intermediate 2, and St Margaret Mary's would offer physics at Higher, and so on.
The schools also hope to foster closer relationships between the two staffs and with the local community by creating an ethos that people can take pride in.
The schools are 10 minutes' walk from each other; one is denominational, the other is non-denominational. Yet both heads agree that the differences between pupils tend not to be along sectarian lines so much as territorial affiliations, and that is a challenge they will tackle jointly.
Both schools already have strong links with the private sector: St Margaret Mary's has been linked with Deloitte and Touche for more than eight years through a mentoring scheme which encourages representatives of the business world to mentor middle school pupils and encourage them to be more ambitious and go on to higher education.
Castlemilk High has links with ScottishPower through a leadership programme which allows Mr McAlinden to meet regularly with Ian Russell, the company's chief executive, and through a skills to work programme for staff.
Among the aims of this joint bid is to offer better professional development for teachers, employ an extra attendance officer who would be shared across both schools and enhance enterprise education activities. "We also want to explore how we can work on formative assessment and take this forward in a sustained systematic way," Pat Scanlan, head of St Margaret Mary's, said.
Mr McAlinden said: "You don't have to be sick to get better. We hope that we will be able to take both schools to a higher level."
The other successful bidders will each receive an additional pound;100,000 a year. They are:
* Anderson High, Lerwick - aims to be a global learning community facilitated by ICT, with twinned schools in South Africa, Japan, Germany, Sweden and the Czech Republic.
* Arbroath Academy - wants to transform pupil motivation by improving the environmental ethos of the school and increasing vocational opportunities.
* Barrhead High, East Renfrewshire - will focus on arts, creativity and language, with artists in residence. The school will provide a lunchtime health clinic organised by pupils.
* Blairgowrie High - will give "more choice, more chance". An extended learning community will include pupils working with local businesses.
* Braes High, Falkirk - will focus on communication skills and technologies and build enterprise activities such as offering media services to associated primaries.
* Braeview Academy, Dundee - will champion creativity and ambition, linked to the performing and visual arts.
* Burnhouse School, Whitburn, which caters for pupils with additional support needs arising from social, emotional and behavioural difficulties - plans its activity-based curriculum around core subjects of English, maths, science and IT.
* Cardinal Newman High, Bellshill - will become an enhanced "enterprise comprehensive" encompassing a hospitality academy.
* Doon Academy Learning Partnership, East Ayrshire, involving the secondary and its five primaries - will offer a flexible and enriched curriculum, as well as a dedicated health counsellor.
* Hawick High - aims to develop enterprise and creativity among pupils, staff, its seven associated primaries, parents and other partners.
* Inverness High - wants to be a centre of the local community, stimulating new, entrepreneurial ways of tackling social problems.
* Islay High - intends to offer a more personalised curriculum, to maximise pupil choice in S3-S6.
* Kirkland High and Community College, Leven, Fife - will develop into a centre of excellence in the creative and performing art.
* Newbattle Community High, Dalkeith - hopes to become a centre of excellence in the arts and this will be used as a catalyst to raise attainment.
* Our Lady's and St Patrick's High, Dumbarton - aims to build on its successes in art, design, and music to achieve its aim of changing lives through creativity.
* St Modan's High, Stirling - plans to extend the school day and offer extended year activities involving the wider community.
* St Ninian's High, Kirkintilloch - plans to develop its expertise in modern languages and to involve parents more through the use of ICT.
* St Paul's High, Glasgow - will focus on preparing pupils for work, enhancing employability skills and learning beyond the classroom.
* Wallace High, Thornhill, Dumfries and Galloway - will restructure its curriculum to offer pupils "pace and challenge", enable smaller class sizes in the lower school and provide more choice and better links to further and higher education.