These fares you have quoted are "London Midland Only" fares. Even though LM are slow (an average of 3 hours compared to Virgin Trains' average of 1.5 hours), they are exceptionally good value, and cannot be reduced further by splitting.
(Some people may suggest other tickets which might save a pound or two, but as these forbid ending your journey "short" at Crewe, cannot be used legitimately.)
Unfortunately, these "London Midland only" fares do not seem to suit your requirements. The last LM train on the return journey leaves Crewe at [17:33] and arrives in Watford at [20:31], which is much earlier than you want.
If you don't mind sticking to a specific train, I would suggest buying "Advance" tickets. You have multiple options on the outward and return journeys:
Outward journey:
- £18.00 Watford [05:45] to Crewe [07:22] (Virgin Trains - 1h 37m)
- £25.00 Watford [06:02] to Crewe [07:30] (Virgin Trains - 1h 28m)*
- £11.50 Watford [06:41] to Crewe [09:38] (London Midland - 2h 57m)*
*Note that First Class is £36.50 on the highlighted Virgin service (just £11.50 more) for which you will be served a full breakfast and get access to the First Class lounge at Crewe. On the LM service First Class is £18 (just £6.50 more) but the only benefit is a quieter compartment - the seats are usually the same as in Standard Class.
Return journey:
- £18.00 Crewe [20:23] to Watford [21:48] (Virgin Trains - 1h 25m)
- £12.00 Crewe [21:24] to Watford [23:30] (Virgin Trains - 2h 06m)
I, personally, would go for the outward LM service at £11.50 and the return VT service at £12. Alternatively, if I wanted a more relaxing journey, I would go for the outward VT service in First Class at £36.50 and the return VT service at £12.