orders can be combined, but as far as i know that is at LUG level not member level, so LUGs process members orders as normal, members can still only participate in one of the schemes and then LUGs order together combining their orders to one large one. i'm guessing both LUGs would need to have the same part selection as the limit per order is 85 different parts. this is for smaller LUGs who wouldn't have the required number of people participating.
You can order along with any LUG you are a member of as long as you only order from one of them each year, but you are not tied to any of them from year to year.
I highly recommend anyone who builds MOCs to participate, there is a maximum spend per person which is lower than you might think so the issue is not how are you going to spend enough to allow the LUG as a whole to participate, but how many parts are you going to take away from your order to get back down below the maximum spend limit. a LUG needs 21 people doing this (Maxing out their order), or more if people are not maxing out their spend limits or no one gets anything. It's important to remember that registering to participate does not mean you are required to order, nor are you required to max out your orders spend limit. so the more people registered to take part the more chances there are of everyone getting to order nice cheap lego direct from the factory ;)