If I am doing Waterbury’s 10x3 can I split the 10x3 and 4x6 into seperate days, so that mon, wed, fri I start the day, then I finish the day 1 workout on the next day.
Eg mon 10x3 bench
tues romanian deads, and all the rest.
wed 10x3 squats
thurs all the rest of wed
fri 10x3 chins
sat rest of fridays workout
Will this still be as effective, or do I REALLY need to do it all one workout?
If you do this then it wouldn’t be Waterbury’s 10x3, now would it? 
The reality is that you can do whatever you want as long as you’re getting the results you want. Just know that if you split the workout then you are not getting the intended effect.
The intended effect is getting effective hypertrophy, but keeping the relative strength of your body (or increasing it if haven’t done strength work before).
Now, because strength is best worked at high frequency I presumed it was not a bad thing if there was 6 days a week training. Overtraining isnt a problem because I am still doing the same volume as pescribed.
The question is would I be under-resting (aka my available energy stores wouldnt be restored by the next workout thus limiting the intensity at which I could do each excercise)?
Or, would the higher frequency interrupt the growing mechanisms (hypertrophy) from stress hormones being released so often?
Is there any reason not to?
Golden Rule: Don’t fuck with it.
Well, seen as Waterbury is designing his programs to cater for the largest number of people, he would be designing it in the spirit of people who want to do full body training 3 times a week. They normally do full-body ONLY because they dont want to or cant go 4-5 times a week. There arent many people who are able to go every day, but only for short amount of time (20-30 mins).
As long as you take a day off each week and do adjacent days with completely different exercises you should be alright. In your example you are following Romanian deads with squats. This is not a good idea. It would be better if adjacent days worked different metabolic systems like Crossfit does.
Dude is right though, you wouldn’t be doing the program, you would be doing your own. That’s OK, your own may suit your needs more.