How do I improve heating in back room of house?
I have a couple of long runs too and I used a fan until I upgraded the HVAC unit.  Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.  

How to measure air flow?  Well pick up an Air Flow Meter.   However I have a suggestion before you spend $$$ on this.  Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents.  Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation.  If you are confident with the bag results and they are obvious then skip the pricy tool.  ##Side Note##  (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow.  You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count.  But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag!  But it does involve slow-motion so my solution is more practical)

I would also take temperature readings inside each duct while on after 5-10 minutes.  How cold is the air you are getting?  

If you are getting comparable air flow and temp you can add another duct or bigger.  Really up to you.  

If you aren't and I am guessing you aren't then you need to first get rid of the flex lines.  They squash air flow, they are affected by outside temperatures more, and they leak.  Put rigid metal ducts in.  If the ducts are in the attic then you need to insulate the crap out of them.  

Then if you still have issues you need to measure air flow and temp again.  If air flow is poor you are looking at a booster.  If it is relatively the same, add another duct.

Then you are talking about adding a return to the room and over-insulating the attic above the room.