When I looked at it, I would say it is tight. Figure basically an hour to an hour and a half just to take the RER, including some risk for waiting for the next train, time to walk from RER station to Station, etc.
That leaves an Hour and a half to get out of CDG, most would say that is optimistic, I have done it in much less, but I have had an hour and a half just in Immigration as well.
Maybe compare the cost of a full price train ticket vs buying when you get there, consider the added cost insurance. I do not think that those trains are likely to sell out.
You could maybe trim off some time by taking a Cab from the Airport, at least it would be direct, for 50 euro, so not much more expensive for two.