Splitting at Stamford saves a little bit, but not huge amounts:
£14 each way on advance tickets (£7 Birmingham to Stamford, £7 Stamford to Cambridge), but still at the same departure times on Friday.
Or £48.50 on an off-peak return (£27.30 Birmingham to Stamford, £21.20 Stamford to Cambridge).
You can get some prices cheaper than £48.50 for different departure times on Friday by getting an advance for one of the legs, and an open return for the other: e.g. departing at 13:22, you can get the £7 advance for Birmingham to Stamford, but you'll have to get the £21.20 fare from Stamford to Cambridge, which adds up to £35.20 in total.