I kind of think they were alone in this.
Otherwise, why did they need to car jack that guy and steal $800 from his ATM?
If they had help, that would be planned out ahead. Right?
I’m not going to find the various articles for you that have been posted here, but they regularly drove Porshe’s and BMW’s prior to the Boston massacre, and bought several guns, lots of ammo, explosives, and the older bro managed to take a trip “home” for 6 months.
No, I don’t think they planned to be caught. Obviously, they were shocked when the FBI released their photos.
Here's a theory as good or as bad as anyone else can come up with. Joker and Tammy became radicalized. They decided on their own to bomb Boston, and they could have told other haters of America of their plans, but they did need help assembling the pressure cooker bombs. We might never find out if they learned it on line or if someone assembled the bombs for them. After the bombing, they thought they were in the clear, and might have had further plans for bombing New York, until the initial photos came out, and they knew they were going to get caught.
Now, being spooked, they fled. Why the killed the MIT officer is still unknown, unless they wanted to take his guns. (His pistol and vehicle mounted shotgun.) They may have done the hijacking to provide them with a different vehicle and some money from the victim. Possibly they were planning to kill the victim before anyone knew the victim and his SUV were missing. Unfortunately for them, the victim escaped, and they knew that the hijacked vehicle would be extremely "hot" because they told the victim they did the Boston bombing.
In spite of all of this, they could still have been thinking they could make it to New York to cause more deaths and injuries, but their luck ran out, especially when Joker ran over Tammy. Joker kept running, and hid in a boat. His plans for escape ended when the boat owner stepped outside for a cigarette.