The Red Rose 50 organisers are consistent with there dates, the middle of September so quite rightly the walk will be held on the 14/15th September.
The Shotley 50 was held in the first weekend of September in 2017 which allowed everyone to do both that month or indeed three if you include the Surrey Tops. However, as a section of the Shotley is governed by the tide, this year it has to be on the 14/15th September.

I agree 100% : there was a post about this earlier in the year :

https://www.ldwa.org.uk/forum/show_topic.php?tpc=2696&dir=S

Fantastic to have so much choice but frustrating for event junkies to miss out thru date clashes then have quiet weekends with no events !

Someone mentioned this on a walk over the weekend but it didn't really sink in until I looked for suitable challenge events later in the year.

Red Red Rose 50
Shotley Peninsula 50
Both on Sat 14th - Sun 15th Sep 2019

Laich o' Moray 50
Sussex Stride 50
Both on Sat 21st - Sun 22nd Sep 2019

Neither of those weekends appear to be bank holidays so I'm at a loss as to why they aren't spread out a bit more. I appreciate they're arranged by different groups and not in the same area, but speaking only for myself I'd have happily gone for both the Laich o' Moray (having travelled quite cheaply to it last year) and the Sussex Stride.

Now I'll have to choose, and on top of that will have walked a 50 the weekend before as I'm already booked on the Shotley. Without recourse to any unhelpful 'we booked ours first' conversations is there a way to encourage some joined up thinking about scheduling events in the future?