This is my opinion only.
I sense that if you use Migration Assistant, it's going to get pretty "confused" trying to migrate data from TWO computers into one (if that's possible at all).
I would recommend:
1. Install your apps first, manually. If you have no way to install apps other than via "migration", looks like you'll have to choose one of the two previous macbooks and go with that one only. You can set up MA so that only "apps" are migrated.
2. I'll guess you could do the same with your old account (if you wish to keep it) and your data -- that is, use MA with only one of the previous macbooks, and "do the other one manually".
3. TIP about migrating items in "home" folder:
You cannot simply copy the entire home folder from one mac to another. Won't work.
BUT -- you CAN copy the contents of folders -inside- the home folder.
EXAMPLE: you can open the "Pictures" folder in the home folder, and copy items one-at-a-time (or a group of items) from that home folder into the Pictures folder on your new Mac, and the pics will "come through" fine.
You may have to be careful about moving items like the "iPhoto library" from one home folder to another (these are actually folders with normally-hidden contents).
What else you may need:
- Pencil
- Writing pad for notes
- Some forethought...