If I have a two entry tourist visa with a date of issue of October 10th. It says "Enter before Jan 11 2015". I already did my first entry and I am in Thailand now.

So does this mean that to activate the second period of almost three months (2 months + extension), I should leave the country before Jan 11 and return before Jan 11?

Yes, you must enter Thailand on or before 11th Jan 2015 in order to activate your second entry.

That is correct. You must use the 2nd entry on or before January 11th.

So does this mean that to activate the second period of almost three months (2 months + extension), I should leave the country before Jan 11 and return before Jan 11?\

You must leave country on or before the "allowed until" date. That may not be Jan 11th if you entered country immediately after visa was issued.

So does this mean that to activate the second period of almost three months (2 months + extension), I should leave the country before Jan 11 and return before Jan 11?\

You must leave country on or before the "allowed until" date. That may not be Jan 11th if you entered country immediately after visa was issued.

Yes, you need to leave Thailand on or before the current extension of stay date you were given by immigrations (stamped in your passport at Immigrations when you did the extension), which as Paz says might be before or after Jan 11.

You have to be back in Thailand on or before the Jan 11 date to activate the second entry and new permission to stay.

Edited October 22, 201411 yr by Suradit69