I'm wondering if this is a SE-centric thread. I'm in Yorkshire, last year I only put down two choices for DS - both out of catchment. I got my first choice
No, this isn't SE-centric. The rules are the same everywhere in England. You would have got your first choice even if you had named more preferences. In an area where schools generally have spare places using all choices is less important but it can still matter. Some people get caught out when they only name 1 or 2 schools that "always" have places only to find that the school gets more applicants than normal.
I don't want my DT to go to the same school. Will they still get placed together even though its not in their best interest?
If you put different preferences for them they will probably end up at different schools. The LA cannot decide to put them together just because they are twins.
How far does it going being a previously lac child
As a previously looked after child, your child is higher priority than anyone else apart from other looked after/previously looked after children. The only children admitted before LAC are those with an EHCP naming the school. The only exception to this is faith schools where some (Catholic schools in particular) only give top priority to LAC of the faith. Unless your first preference is one of those, your child is pretty much guaranteed a place. You can ignore distance. That will only come into play in the unlikely event that there are more LAC applying than places.
I really don't want either child to go to the closet school for many reasons, they have an entry test so by not doing the entry test can they not be allocated a place there
I presume your closest school is a grammar school. If the entry test has a pass mark your child cannot be allocated a place there if they don't sit the test. However, if it is simply used to determine priority with no specific pass mark your child could, in theory, be allocated a place, although it is unlikely. However, as you say your child is formerly looked after this is irrelevant. You will be offered your first preference.
Can anyone link me to where you find the data for how many applicants the school received last year and how many metres the furthest successful applicant lived
No-one can give you a link unless you say where you live. The information will be in the school admissions section of your local council's website.