Dual extruder clogging problem I have attached a dual extruder (2 in 2 out) to my 3D printer. There is a repeating problem of clogging while printing.

I have fixed Chimera hotend. 



A big fan, which covers area more than the double extruder, cools both cold ends. An additional fan directly on the front (as seen in the picture) cold end cools the ribs directly. The back does not have an additional fan.

I tried to print after attaching an extra fan; the first print went well but from there on, the hotend gets clogged every time I print.

The clogging happens in melt zone. I have tried cleaning the hot-end, but to no avail...

My settings are as below:


Print speed = 60mm/sec
Temperature = 210C
Filament material = PLA
Filament diameter = 1.75,
Nozzle Diameter = 0.4
Dual extrusion switch amount = 16.5mm
Retraction speed = 40mm/sec,
Retraction Distance = 4.5mm,
Disabled Wipe and Prime tower and ooze shield


Please advise what to do.
It appears (now confirmed in your question) that you are using a Chimera 2-in-2-out hotend like depicted below:



Chimera hotends generally use all-metal heat breaks (not confirmed yet) which are harder to operate than "normal" PTFE lined hotends. A common problem that happens is that heat creeps up causing problems as a result of too slow printing or too high retraction length settings. Basically your problem is clogging related to all-metal heat break printing. This is recently asked in this question: "Apparently systematic nozzle clogging". Please read the answers to that question as they describe very well what happens and what to do.

Quoting from this article explains that it is a heating/cooling issue: 


  Jams and clogs are often from a combination of excessive heat and
  non-optimal material flow.  This effect is worsened by poorly cooled
  all-metal hot ends, high torque extruder gears, small nozzles/layers,
  slow printing speeds, too thin first layer, and excessive retraction.


Please do read this article and the solutions they mention to get yourself acquainted with the problem and its solutions. 

In your case this could well be a cooling issue in combination with a too high print temperature and a possibly too high retraction length setting (depends on the length of the Bowden tube). The fan you mounted in the front (onto the cooling ribs, so the rightt fan in your image) is too large for this hotend. You attached the fan to the left top mounting hole so that it exceeds the right and bottom area of your cooling ribs. Furthermore, this larger fan has a larger center (hub) to house the bigger engine so that you end up with a lower cooling flow for at least the right extruder as most of the cooling ribs area is blocked by the fan hub. Please fit a fan that fits this hotend. Printing a fan adaptors may not work as well, these fans have a very low static pressure difference, so a convergent duct may not work as well as you might think. Considering your temperature you are printing PLA; PLA could usually be printed well in the 185 - 195 &deg;C temperature range (depending on the PLA), 210 &deg;C seems a bit on the high side. 

Another solution could be replacing the heat breaks for lined heat breaks:




As an aside, to explain a suggestion, in Himanshu's comment to coat your filament with some oils:


  Apply some oil on PLA filament with cotton.


Oil does lower friction in an all-metal hotend, but please beware that this needs to be vegetable oil. The reasoning behind this is that the oil hardens inside the throat to create a smooth coating on the inside, not lube the inside of the throat. This thread has some interesting material on that. 

However, general consensus is not to do that:


  Micro-Swiss use slippery platings and such, they don't need it. I've
  also used B3 Picos and E3D v5, v6, Volcano and Chimera and even that
  one garbage QUBD mk7 knockoff. Never needed oil.


or


  Seasoning hotends is a myth that refuses to die. Just don't do it, it
  does more harm than good.