I’ve heard that the best way to do shnayim mikra is to do the mikrah twice then the targum, but bidieved it’s okay to do the mikra (once) and targum, then do the other mikra during kriyas hatorah in Shul. If that’s correct, for someone who would spend that time (the time it takes to do the mikra a second time) learning gemara, would it be best do the second way which is generally bideved but leads to more learning, or the first way even though ultimately it would lead to less torah learning?
What you heard is written in the Mishna Berura 285-6, and indeed it is better to read the mikra twice and then the targum. As far as bitul Torah is concerned, there is no way we can call fulfilling the halacha, which tells us to read from the Torah- bitul Torah! The gemora says that a person that reads shnayim mikra and once targum gets his days and years lengthened. I once heard (I think, in the name of R’ Shteinman zt”l) that there are two mitzvos where the idea of arichus yamim is mentioned, where people abstain from doing the mitzva because they feel that it is wasting their time, Kibbud Av v’em and Shanayim Mikra. Therefore, the Torah tells us, “don’t worry” you are not wasting your time, because I (Hashem) is the one who gives life and you will be paid back for the investment in my mitzvos, and you will be given extra time, for the time that you think you wasted. (See sources for other paces this idea is mentioned).
You should be zoche to fulfill the mitzva of Shanaim Mikra properly and have aricus yamim.
Brachos 8a. Regarding arichus yamim see Kriyas Shanim Mikra Vechod Targum Kihlchoso pg. 20 ftnt. 6 in the name Divrei Chaver ben Chaim (a talmid of the Chasam Sofer) Chulin Chap. 2, This idea is also mentioned in Arugas Habosem Parshas Tzav. I think I also heard this from R’ Shteiman zt”l.