im travelling off peak from Stoke on Trent to Blackpool North then returning two days later. Is it cheaper to split at Manchester?
That would depend entirely on whether you're making your outward journey immediately or in advance.
However, as the cheapest return fare is £41.50 (priced by either Virgin, or a lunatic, or both) a through ticket isn't really value for money.
Assuming no railcards, Northern advance fares (specific train, no refunds) for Stoke-Manc can be had for £3.00 each way, or £13.10 for an Off Peak Return valid on any Virgin Train after 9.30am, or any train company after 9.30am costing £16.30.
Northern advances between Manchester and Blackpool are available from £5.00 each way, or £18.20 for a flexible Off Peak Return.
An Off Peak Return allows you to make your outward journey on the first day, and your return journey within a month.
Depending on your budget, and how willing you are to commit to specific trains, the entire return journey could be done for as little as £16.00 on Advance fares, or on flexible tickets for approximately twice that.
Alternatively, if you can get your hands on a PC, printer and some JP/Trinity newspapers, buy a Northern Rail Only day ticket for £10 per day (ie £20 return), details
here, and you would be able to take any Northern train on weekends, or after 0845 M-F.
Edit: It would also be helpful if you could change the thread title to something more accurate, perhaps reflecting the fact that you're travelling to Blackpool, not Manchester.